diff --git a/src/fe.c b/src/fe.c
index 58b0380..5ff8e5f 100644
--- a/src/fe.c
+++ b/src/fe.c
@@ -765,6 +765,74 @@ void fe_mul(fe h, const fe f, const fe g) {
 }
 
 
+/*
+h = f * 121666
+Can overlap h with f.
+
+Preconditions:
+   |f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.
+
+Postconditions:
+   |h| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.
+*/
+
+void fe_mul121666(fe h, fe f) {
+	int32_t f0 = f[0];
+	int32_t f1 = f[1];
+	int32_t f2 = f[2];
+	int32_t f3 = f[3];
+	int32_t f4 = f[4];
+	int32_t f5 = f[5];
+	int32_t f6 = f[6];
+	int32_t f7 = f[7];
+	int32_t f8 = f[8];
+	int32_t f9 = f[9];
+	int64_t h0 = f0 * (int64_t) 121666;
+	int64_t h1 = f1 * (int64_t) 121666;
+	int64_t h2 = f2 * (int64_t) 121666;
+	int64_t h3 = f3 * (int64_t) 121666;
+	int64_t h4 = f4 * (int64_t) 121666;
+	int64_t h5 = f5 * (int64_t) 121666;
+	int64_t h6 = f6 * (int64_t) 121666;
+	int64_t h7 = f7 * (int64_t) 121666;
+	int64_t h8 = f8 * (int64_t) 121666;
+	int64_t h9 = f9 * (int64_t) 121666;
+	int64_t carry0;
+	int64_t carry1;
+	int64_t carry2;
+	int64_t carry3;
+	int64_t carry4;
+	int64_t carry5;
+	int64_t carry6;
+	int64_t carry7;
+	int64_t carry8;
+	int64_t carry9;
+
+	carry9 = (h9 + (int64_t) (1<<24)) >> 25; h0 += carry9 * 19; h9 -= carry9 << 25;
+	carry1 = (h1 + (int64_t) (1<<24)) >> 25; h2 += carry1; h1 -= carry1 << 25;
+	carry3 = (h3 + (int64_t) (1<<24)) >> 25; h4 += carry3; h3 -= carry3 << 25;
+	carry5 = (h5 + (int64_t) (1<<24)) >> 25; h6 += carry5; h5 -= carry5 << 25;
+	carry7 = (h7 + (int64_t) (1<<24)) >> 25; h8 += carry7; h7 -= carry7 << 25;
+
+	carry0 = (h0 + (int64_t) (1<<25)) >> 26; h1 += carry0; h0 -= carry0 << 26;
+	carry2 = (h2 + (int64_t) (1<<25)) >> 26; h3 += carry2; h2 -= carry2 << 26;
+	carry4 = (h4 + (int64_t) (1<<25)) >> 26; h5 += carry4; h4 -= carry4 << 26;
+	carry6 = (h6 + (int64_t) (1<<25)) >> 26; h7 += carry6; h6 -= carry6 << 26;
+	carry8 = (h8 + (int64_t) (1<<25)) >> 26; h9 += carry8; h8 -= carry8 << 26;
+
+	h[0] = h0;
+	h[1] = h1;
+	h[2] = h2;
+	h[3] = h3;
+	h[4] = h4;
+	h[5] = h5;
+	h[6] = h6;
+	h[7] = h7;
+	h[8] = h8;
+	h[9] = h9;
+}
+
+
 /*
 h = -f
 
@@ -796,6 +864,7 @@ void fe_neg(fe h, const fe f) {
     int32_t h7 = -f7;
     int32_t h8 = -f8;
     int32_t h9 = -f9;
+
     h[0] = h0;
     h[1] = h1;
     h[2] = h2;
diff --git a/src/fe.h b/src/fe.h
index 8f6fb62..b4b62d2 100644
--- a/src/fe.h
+++ b/src/fe.h
@@ -34,6 +34,7 @@ void fe_invert(fe out, const fe z);
 void fe_sq(fe h, const fe f);
 void fe_sq2(fe h, const fe f);
 void fe_mul(fe h, const fe f, const fe g);
+void fe_mul121666(fe h, fe f);
 void fe_pow22523(fe out, const fe z);
 void fe_sub(fe h, const fe f, const fe g);