Samples of four species of algae were collected from three mid-shore pools on each of three shores on the north-east coast of England in order to describe colour polymorphism in the common intertidal ostracod Paradoxostoma variabile. Eight different colour morphs were found within the populations. On all shores, male P. variabile occurred in significantly higher densities than did the females, and males exhibited a significantly higher colour morph diversity and colour morph richness than females. On two out of three shores, males showed a significant association in colour morph frequency with algal species (but the pattern was not consistent between shores), but no such association was found in the female populations. In either sex, colour morph diversity did not vary between algal species but samples of male ostracods from the shore at Holbeck contained a significantly higher diversity of colour morphs than samples from the other two shores. In the laboratory, neither sex demonstrated a specific association with algal species suggesting that habitat selection is not an important factor in maintaining colour morph frequencies in this species.