diff --git a/include/chibi/features.h b/include/chibi/features.h
index ccf27b04..09926179 100644
--- a/include/chibi/features.h
+++ b/include/chibi/features.h
@@ -303,6 +303,43 @@
 #endif
 #endif
 
+/* Detect specific BSD */
+#if SEXP_BSD
+#if defined(__APPLE__)
+#define SEXP_DARWIN 1
+#define SEXP_FREEBSD 0
+#define SEXP_NETBSD 0
+#define SEXP_DRAGONFLY 0
+#define SEXP_OPENBSD 0
+#elif defined(__FreeBSD__)
+#define SEXP_DARWIN 0
+#define SEXP_FREEBSD 1
+#define SEXP_NETBSD 0
+#define SEXP_DRAGONFLY 0
+#define SEXP_OPENBSD 0
+#elif defined(__NetBSD__)
+#define SEXP_DARWIN 0
+#define SEXP_FREEBSD 0
+#define SEXP_NETBSD 1
+#define SEXP_DRAGONFLY 0
+#define SEXP_OPENBSD 0
+#define SEXP_NETBSD 0
+#elif defined(__DragonFly__)
+#define SEXP_DARWIN 1
+#define SEXP_FREEBSD 0
+#define SEXP_NETBSD 0
+#define SEXP_DRAGONFLY 1
+#define SEXP_OPENBSD 0
+#define SEXP_DRAGONFLY 0
+#elif defined(__OpenBSD__)
+#define SEXP_DARWIN 0
+#define SEXP_FREEBSD 0
+#define SEXP_NETBSD 0
+#define SEXP_DRAGONFLY 0
+#define SEXP_OPENBSD 1
+#endif
+#endif
+
 /* for bignum support, need a double long to store long*long */
 /* gcc supports uint128_t, otherwise we need a custom struct */
 #ifndef SEXP_USE_CUSTOM_LONG_LONGS
diff --git a/sexp.c b/sexp.c
index cc9c6fc2..8ceef0e1 100644
--- a/sexp.c
+++ b/sexp.c
@@ -464,6 +464,21 @@ static const char* sexp_initial_features[] = {
 #if SEXP_BSD
   "bsd",
 #endif
+#if SEXP_DARWIN
+  "darwin",
+#endif
+#if SEXP_OPENBSD
+  "openbsd",
+#endif
+#if SEXP_FREEBSD
+  "freebsd",
+#endif
+#if SEXP_NETBSD
+  "netbsd",
+#endif
+#if SEXP_DRAGONFLY
+  "dragonfly",
+#endif
 #if defined(_WIN32)
   "windows",
 #endif