diff --git a/htslib/kstring.h b/htslib/kstring.h
index 53a19806d..0a3efb7d2 100644
--- a/htslib/kstring.h
+++ b/htslib/kstring.h
@@ -375,17 +375,63 @@ static inline int kputw(int c, kstring_t *s)
 
 static inline int kputll(long long c, kstring_t *s)
 {
-	char buf[32];
-	int i, l = 0;
-	unsigned long long x = c;
-	if (c < 0) x = -x;
-	do { buf[l++] = x%10 + '0'; x /= 10; } while (x > 0);
-	if (c < 0) buf[l++] = '-';
-	if (ks_resize(s, s->l + l + 2) < 0)
-		return EOF;
-	for (i = l - 1; i >= 0; --i) s->s[s->l++] = buf[i];
-	s->s[s->l] = 0;
-	return 0;
+    // Worst case expansion.  One check reduces function size
+    // and aids inlining chance.  Memory overhead is minimal.
+    if (ks_resize(s, s->l + 23) < 0)
+	return EOF;
+
+    unsigned long long x = c;
+    if (c < 0) {
+	x = -x;
+        s->s[s->l++] = '-';
+    }
+
+    if (x <= UINT32_MAX)
+	return kputuw(x, s);
+
+    static const char kputull_dig2r[] =
+        "00010203040506070809"
+        "10111213141516171819"
+        "20212223242526272829"
+        "30313233343536373839"
+        "40414243444546474849"
+        "50515253545556575859"
+        "60616263646566676869"
+        "70717273747576777879"
+        "80818283848586878889"
+        "90919293949596979899";
+    unsigned int l, j;
+    char *cp;
+
+    // Find out how long the number is (could consider clzll)
+    uint64_t m = 1;
+    l = 0;
+    if (sizeof(long long)==sizeof(uint64_t) && x >= 10000000000000000000ULL) {
+	// avoids overflow below
+	l = 20;
+    } else {
+	do {
+	    l++;
+	    m *= 10;
+	} while (x >= m);
+    }
+
+    // Add digits two at a time
+    j = l;
+    cp = s->s + s->l;
+    while (x >= 10) {
+        const char *d = &kputull_dig2r[2*(x%100)];
+        x /= 100;
+        memcpy(&cp[j-=2], d, 2);
+    }
+
+    // Last one (if necessary).  We know that x < 10 by now.
+    if (j == 1)
+        cp[0] = x + '0';
+
+    s->l += l;
+    s->s[s->l] = 0;
+    return 0;
 }
 
 static inline int kputl(long c, kstring_t *s) {
diff --git a/test/test_kstring.c b/test/test_kstring.c
index feb8243df..f942656f1 100644
--- a/test/test_kstring.c
+++ b/test/test_kstring.c
@@ -261,6 +261,84 @@ static int test_kputw(int64_t start, int64_t end) {
     return 0;
 }
 
+static int test_kputll_from_to(kstring_t *str, long long s, long long e) {
+    long long i = s;
+
+    for (;;) {
+        str->l = 0;
+        memset(str->s, 0xff, str->m);
+        if (kputll(i, str) < 0 || !str->s) {
+            perror("kputll");
+            return -1;
+        }
+        if (str->l >= str->m || str->s[str->l] != '\0') {
+            fprintf(stderr, "No NUL termination on string from kputll\n");
+            return -1;
+        }
+        if (i != strtoll(str->s, NULL, 10)) {
+            fprintf(stderr,
+                    "kputll wrote the wrong value, expected %lld, got %s\n",
+                    i, str->s);
+            return -1;
+        }
+        if (i >= e) break;
+        i++;
+    }
+    return 0;
+}
+
+static int test_kputll(long long start, long long end) {
+    kstring_t str = { 0, 0, NULL };
+    unsigned long long val;
+
+    str.s = malloc(2);
+    if (!str.s) {
+        perror("malloc");
+        return -1;
+    }
+    str.m = 2;
+
+    for (val = 1; val < INT64_MAX-5; val *= 10) {
+        if (test_kputll_from_to(&str, val >= 5 ? val - 5 : val, val) < 0) {
+            free(ks_release(&str));
+            return -1;
+        }
+    }
+
+    for (val = 1; val < INT64_MAX-5; val *= 10) {
+        long long valm = -val;
+        if (test_kputll_from_to(&str, valm >= 5 ? valm - 5 : valm, valm) < 0) {
+            free(ks_release(&str));
+            return -1;
+        }
+    }
+
+    if (test_kputll_from_to(&str, INT64_MAX - 5, INT64_MAX) < 0) {
+        free(ks_release(&str));
+        return -1;
+    }
+
+    if (test_kputll_from_to(&str, INT64_MIN, INT64_MIN + 5) < 0) {
+        free(ks_release(&str));
+        return -1;
+    }
+
+    str.m = 1; // Force a resize
+    int64_t start2 = (int64_t)start; // no larger on our platforms
+    int64_t end2   = (int64_t)end;
+    clamp(&start2, INT64_MIN, INT64_MAX);
+    clamp(&end2,   INT64_MIN, INT64_MAX);
+
+    if (test_kputll_from_to(&str, start, end) < 0) {
+        free(ks_release(&str));
+        return -1;
+    }
+
+    free(ks_release(&str));
+
+    return 0;
+}
+
 // callback used by test_kgetline
 static char *mock_fgets(char *str, int num, void *p) {
     int *mock_state = (int*)p;
@@ -413,6 +491,9 @@ int main(int argc, char **argv) {
     if (!test || strcmp(test, "kputw") == 0)
         if (test_kputw(start, end) != 0) res = EXIT_FAILURE;
 
+    if (!test || strcmp(test, "kputll") == 0)
+        if (test_kputll(start, end) != 0) res = EXIT_FAILURE;
+
     if (!test || strcmp(test, "kgetline") == 0)
         if (test_kgetline() != 0) res = EXIT_FAILURE;