diff --git a/DESCRIPTION b/DESCRIPTION
index eb1b244c..4975d5b1 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: monty
 Title: Monte Carlo Models
-Version: 0.2.19
+Version: 0.2.20
 Authors@R: c(person("Rich", "FitzJohn", role = c("aut", "cre"),
                     email = "rich.fitzjohn@gmail.com"),
              person("Wes", "Hinsley", role = "aut"),
diff --git a/R/dsl-distributions.R b/R/dsl-distributions.R
index acd4576d..048e7f3c 100644
--- a/R/dsl-distributions.R
+++ b/R/dsl-distributions.R
@@ -125,6 +125,34 @@ distr_hypergeometric <- distribution(
     mean = quote(k * n1 / (n1 + n2))),
   cpp = list(density = "hypergeometric", sample = "hypergeometric"))
 
+distr_negative_binomial_mu <- distribution(
+  name = "NegativeBinomial",
+  variant = "mu",
+  density = function(x, size, mu) {
+    dnbinom(x, size, mu = mu, log = TRUE)
+  },
+  domain = c(0, Inf),
+  sample = function(rng, size, mu) rng$negative_binomial_mu(1, size, mu),
+  expr = list(
+    density = quote(lchoose(x + size - 1, x) + size * log(size) + 
+                      x * log(mu) - (size + x) * log(size + mu)),
+    mean = quote(mu)),
+  cpp = list(density = "negative_binomial_mu", sample = "negative_binomial_mu"))
+
+distr_negative_binomial_prob <- distribution(
+  name = "NegativeBinomial",
+  variant = "prob",
+  density = function(x, size, prob) {
+    dnbinom(x, size, prob = prob, log = TRUE)
+  },
+  domain = c(0, Inf),
+  sample = function(rng, size, prob) rng$negative_binomial_prob(1, size, prob),
+  expr = list(
+    density = quote(lchoose(x + size - 1, x) + x * log(1 - prob) + 
+                      size * log(prob)),
+    mean = quote(size * (1 - prob) / prob)),
+  cpp = list(density = "negative_binomial_prob", sample = "negative_binomial_prob"))
+
 distr_normal <- distribution(
   name = "Normal",
   density = function(x, mean, sd) dnorm(x, mean, sd, log = TRUE),
@@ -166,6 +194,8 @@ dsl_distributions <- local({
     distr_gamma_rate,
     distr_gamma_scale,
     distr_hypergeometric,
+    distr_negative_binomial_mu,
+    distr_negative_binomial_prob,
     distr_normal,
     distr_poisson,
     distr_uniform)