The reflected patterns and colours of terrestrial animals often show a sexual dimorphism associated with visual display, mate recognition and selection. In the sea the structures associated with bioluminescence may also show a marked sexual dimorphism. Some apparent bioluminescent dimorphisms (e.g. differences in photophore numbers) are probably secondary functions of sexual differences in size. A role in sexual communication is much more likely where specific photophores are developed or enlarged in mature specimens of one sex only but the presence of light organs in female anglerfishes (but not in males) is complicated by a significant size dimorphism. Dimorphisms in dragonfishes and lanternfishes primarily involve the relative enlargement of particular photophores in the males. A sexual role is assumed, but the morphological differences are often small. Most male ponyfishes have enlarged light organs; behavioural observations of free-swimming animals have clearly demonstrated that the males use them to generate bioluminescent sexual signals.