diff --git a/src/gen/distribution/commons_math.clj b/src/gen/distribution/commons_math.clj
index 6d0c0df..2808fef 100644
--- a/src/gen/distribution/commons_math.clj
+++ b/src/gen/distribution/commons_math.clj
@@ -96,12 +96,34 @@
 (defn uniform-discrete-distribution [low high]
   (UniformIntegerDistribution. (rng) low high))
 
-(defn categorical-distribution [probabilities]
+(defn- v->categorical [probabilities]
   (let [n  (count probabilities)
         ks (int-array (range n))
         vs (double-array probabilities)]
     (EnumeratedIntegerDistribution. (rng) ks vs)))
 
+(defn- m->categorical [probabilities]
+  (let [ks   (keys probabilities)
+        vs   (vals probabilities)
+        k->i (zipmap ks (range))
+        i->k (zipmap (range) ks)]
+    (-> (v->categorical vs)
+        (d/->Encoded k->i i->k))))
+
+(defn categorical-distribution
+  "Given either
+
+  - a sequence of `probabilities` that sum to 1.0
+  - a map of object => probability (whose values sum to 1.0)
+
+  returns a distribution that produces samples of an integer in the range $[0,
+  n)$ (where `n == (count probabilities)`), or of a map key (for map-shaped
+  `probabilities`)."
+  [probabilities]
+  (if (map? probabilities)
+    (m->categorical probabilities)
+    (v->categorical probabilities)))
+
 ;; ## Primitive generative functions
 
 (def bernoulli
diff --git a/test/gen/distribution/commons_math_test.clj b/test/gen/distribution/commons_math_test.clj
index 6abacd9..8068278 100644
--- a/test/gen/distribution/commons_math_test.clj
+++ b/test/gen/distribution/commons_math_test.clj
@@ -10,6 +10,9 @@
 (deftest beta-tests
   (dt/beta-tests commons/beta-distribution))
 
+(deftest categorical-tests
+  (dt/categorical-tests commons/categorical-distribution))
+
 (deftest uniform-tests
   (dt/uniform-tests commons/uniform-distribution))
 
diff --git a/test/gen/distribution_test.cljc b/test/gen/distribution_test.cljc
index 8938ba5..f90a3e7 100644
--- a/test/gen/distribution_test.cljc
+++ b/test/gen/distribution_test.cljc
@@ -46,6 +46,25 @@
                          (Math/exp (dist/logpdf (->bernoulli p) (not v)))))
                 "All options sum to 1")))
 
+(defn categorical-tests [->cat]
+  (checking "map => categorical properties"
+            [p (gen-double 0 1)]
+            (let [dist (->cat {:true p :false (- 1 p)})]
+              (is (ish? (Math/log p) (dist/logpdf dist :true))
+                  "prob of `:true` matches `p`")
+
+              (is (ish? (Math/log (- 1 p)) (dist/logpdf dist :false))
+                  "prob of `:false` matches `1-p`")))
+
+  (checking "vector => categorical properties"
+            [p (gen-double 0 1)]
+            (let [dist (->cat [p (- 1 p)])]
+              (is (ish? (Math/log p) (dist/logpdf dist 0))
+                  "prob of `1` matches `p`")
+
+              (is (ish? (Math/log (- 1 p)) (dist/logpdf dist 1))
+                  "prob of `0` matches `1-p`"))))
+
 (defn bernoulli-gfi-tests [bernoulli-dist]
   (testing "bernoulli-call-no-args"
     (is (boolean? (bernoulli-dist))))