“Sex is natural, gender is a social construct.” Or so we’re often told. But what does it mean for sex, or the sexes (male and female), to be natural? One way of construing this statement is to say that they are natural kinds. After presenting and briefly defending the “simple causal theory” of natural kinds, I will use it to determine whether the categories male and female correspond to natural kinds across the animal kingdom. This will entail taking a quick look at some biological details, involving gonads, gametes, and anisogamy. After this whirlwind tour of biology, we should be in a better position to say whether sex is a natural kind. If sex is a natural kind, what, if anything, does this imply about gender? Could sex be a natural kind while gender is not, or vice versa, or could both be natural kinds in different domains? I’ll touch briefly on these questions in conclusion.