diff --git a/configure.ac b/configure.ac
index 9b10e8a3..609b4f13 100644
--- a/configure.ac
+++ b/configure.ac
@@ -141,6 +141,13 @@ AC_ARG_ENABLE(assertions, [  --enable-assertions     enable additional software
   AC_DEFINE([ENABLE_ASSERTIONS], , [Assertions])
 fi])
 
+ac_enable_fuzzing="no"
+AC_ARG_ENABLE(fuzzing, [  --enable-fuzzing     causes the encoder to make random decisions],
+[if test "$enableval" = yes; then
+  ac_enable_fuzzing="yes"
+  AC_DEFINE([FUZZING], , [Fuzzing])
+fi])
+
 if test "$OPUS_BUILD" != "true" ; then
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fvisibility=hidden"
@@ -219,6 +226,7 @@ AC_MSG_RESULT([
       Fixed point debugging: ......... ${ac_enable_fixed_debug}
       Custom modes: .................. ${ac_enable_custom_modes}
       Assertion checking: ............ ${ac_enable_assertions}
+      Fuzzing: ..           .......... ${ac_enable_fuzzing}
 ------------------------------------------------------------------------
 ])
 
diff --git a/libcelt/bands.c b/libcelt/bands.c
index a522b504..68193a6d 100644
--- a/libcelt/bands.c
+++ b/libcelt/bands.c
@@ -477,6 +477,10 @@ int spreading_decision(const CELTMode *m, celt_norm *X, int *average,
    } else {
       decision = SPREAD_NONE;
    }
+#ifdef FUZZING
+   decision = rand()&0x3;
+   *tapset_decision=rand()%3;
+#endif
    return decision;
 }
 
diff --git a/libcelt/celt.c b/libcelt/celt.c
index 77301a11..bbd8ca10 100644
--- a/libcelt/celt.c
+++ b/libcelt/celt.c
@@ -387,6 +387,9 @@ static int transient_analysis(const opus_val32 * restrict in, int len, int C,
          is_transient=1;
    }
    RESTORE_STACK;
+#ifdef FUZZING
+   is_transient = rand()&0x1;
+#endif
    return is_transient;
 }
 
@@ -709,6 +712,12 @@ static int tf_analysis(const CELTMode *m, int len, int C, int isTransient,
          tf_res[i] = path0[i+1];
    }
    RESTORE_STACK;
+#ifdef FUZZING
+   tf_select = rand()&0x1;
+   tf_res[0] = rand()&0x1;
+   for (i=1;i<len;i++)
+      tf_res[i] = tf_res[i-1] ^ ((rand()&0xF) == 0);
+#endif
    return tf_select;
 }
 
@@ -857,6 +866,9 @@ static int alloc_trim_analysis(const CELTMode *m, const celt_norm *X,
       trim_index = 0;
    if (trim_index>10)
       trim_index = 10;
+#ifdef FUZZING
+   trim_index = rand()%11;
+#endif
    return trim_index;
 }
 
@@ -1102,6 +1114,10 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
          CELT_COPY(pre[c]+COMBFILTER_MAXPERIOD, in+c*(N+st->overlap)+st->overlap, N);
       } while (++c<CC);
 
+#ifdef FUZZING
+      if ((rand()&0x3F)==0)
+         silence = 1;
+#endif
       if (tell==1)
          ec_enc_bit_logp(enc, silence, 15);
       else
@@ -1332,10 +1348,19 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
          if (C==2)
             d2 = HALF32(d2 + 2*bandLogE[i+st->mode->nbEBands]-
                   bandLogE[i-1+st->mode->nbEBands]-bandLogE[i+1+st->mode->nbEBands]);
+#ifdef FUZZING
+         if((rand()&0xF)==0)
+         {
+            offsets[i] += 1;
+            if((rand()&0x3)==0)
+               offsets[i] += 1+(rand()&0x3);
+         }
+#else
          if (d2 > SHL16(t1,DB_SHIFT))
             offsets[i] += 1;
          if (d2 > SHL16(t2,DB_SHIFT))
             offsets[i] += 1;
+#endif
       }
    }
    dynalloc_logp = 6;
@@ -1527,6 +1552,9 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
    if (anti_collapse_rsv > 0)
    {
       anti_collapse_on = st->consec_transient<2;
+#ifdef FUZZING
+      anti_collapse_on = rand()&0x1;
+#endif
       ec_enc_bits(enc, anti_collapse_on, 1);
    }
    quant_energy_finalise(st->mode, st->start, st->end, oldBandE, error, fine_quant, fine_priority, nbCompressedBytes*8-ec_tell(enc), enc, C);
diff --git a/libcelt/cwrs.c b/libcelt/cwrs.c
index dfc943e4..0c090f85 100644
--- a/libcelt/cwrs.c
+++ b/libcelt/cwrs.c
@@ -38,6 +38,8 @@
 #include "mathops.h"
 #include "arch.h"
 
+#ifdef CUSTOM_MODES
+
 /*Guaranteed to return a conservatively large estimate of the binary logarithm
    with frac bits of fractional precision.
   Tested for all possible 32-bit inputs with frac=4, where the maximum
@@ -68,6 +70,7 @@ int log2_frac(opus_uint32 val, int frac)
   /*Exact powers of two require no rounding.*/
   else return l-1<<frac;
 }
+#endif
 
 #ifndef SMALL_FOOTPRINT
 
diff --git a/libcelt/cwrs.h b/libcelt/cwrs.h
index 515a4a3c..77123943 100644
--- a/libcelt/cwrs.h
+++ b/libcelt/cwrs.h
@@ -35,7 +35,9 @@
 #include "entenc.h"
 #include "entdec.h"
 
+#ifdef CUSTOM_MODES
 int log2_frac(opus_uint32 val, int frac);
+#endif
 
 void get_required_bits(opus_int16 *bits, int N, int K, int frac);
 
diff --git a/libcelt/rate.c b/libcelt/rate.c
index ecbd9ceb..e196d6a7 100644
--- a/libcelt/rate.c
+++ b/libcelt/rate.c
@@ -351,7 +351,11 @@ static inline int interp_bits2pulses(const CELTMode *m, int start, int end, int
                skip here must be explicitly signaled.*/
             /*Choose a threshold with some hysteresis to keep bands from
                fluctuating in and out.*/
+#ifdef FUZZING
+            if ((rand()&0x1) == 0)
+#else
             if (band_bits > ((j<prev?7:9)*band_width<<LM<<BITRES)>>4)
+#endif
             {
                ec_enc_bit_logp(ec, 1, 1);
                break;
diff --git a/src/opus_encoder.c b/src/opus_encoder.c
index b348d5eb..908022d2 100644
--- a/src/opus_encoder.c
+++ b/src/opus_encoder.c
@@ -217,6 +217,12 @@ int opus_encode(OpusEncoder *st, const opus_int16 *pcm, int frame_size,
     } else {
             st->stream_channels = st->channels;
     }
+
+#ifdef FUZZING
+    if (st->channels == 2 && (rand()&0x1F)==0)
+       st->stream_channels = 3-st->stream_channels;
+#endif
+
     /* Equivalent bit-rate for mono */
     mono_rate = st->bitrate_bps;
     if (st->stream_channels==2)
@@ -225,6 +231,20 @@ int opus_encode(OpusEncoder *st, const opus_int16 *pcm, int frame_size,
        of 60 bits/frame */
     mono_rate -= 60*(st->Fs/frame_size - 50);
 
+#ifdef FUZZING
+    if ((rand()&0xF)==0)
+    {
+       if ((rand()&0x1)==0)
+          st->mode = MODE_CELT_ONLY;
+       else
+          st->mode = MODE_SILK_ONLY;
+    } else {
+       if (st->prev_mode==MODE_CELT_ONLY)
+          st->mode = MODE_CELT_ONLY;
+       else
+          st->mode = MODE_SILK_ONLY;
+    }
+#else
     /* Mode selection depending on application and signal type */
     if (st->user_mode==OPUS_APPLICATION_VOIP)
     {
@@ -260,6 +280,7 @@ int opus_encode(OpusEncoder *st, const opus_int16 *pcm, int frame_size,
         else
             st->mode = MODE_SILK_ONLY;
     }
+#endif
     /* Automatic (rate-dependent) bandwidth selection */
     if (st->mode == MODE_CELT_ONLY || st->first || st->silk_mode.allowBandwidthSwitch)
     {