Samples were collected from six pools in order to examine the distribution, body length, base colour and phenotypic diversity (chromatophore pattern) of Idotea granulosa populations on five different species of algae (Ceramium, Corallina, Chondrus, Fucus and Enteromorpha), on each of two shores. Holbeck (HOL) pools had significantly higher habitat heterogeneity in terms of algal species diversity than did those on the more exposed Filey Brigg (FB). Idotea densities were significantly higher and mean body size significantly smaller on Corallina than the other algal species. Isopods were significantly larger on Fucus. There was no significant difference in base colour frequencies between shores but there was a positive association between the brown base colour and Corallina and the green colour and Fucus. The commonest chromatophore pattern morph was the granulosa-maculata that occurred in 44% of the isopods sampled and five out of ten chromatophore patterns accounted for <10% of the population. The uniformis morph was rare on both shores. There was no significant difference in chromatophore pattern frequency between the sexes. The lineata-maculata morph and maculata morphs showed a positive association with Corallina and Fucus respectively. There was a significant difference in chromatophore pattern frequency between adults and juveniles. The uniformis and maculata patterns occurred in higher frequencies than expected in the adult population whereas the reverse was true of the granulosa-maculata and lineata-maculata morphs. A total of 85 different phenotypes were recorded from both shores, with 51 of these being exclusively found in HOL pools. Holbeck pools had a significantly higher isopod phenotypic diversity than did those from FB. There was no significant difference in phenotypic diversity between adult and juvenile Idotea at HOL, however juveniles had a significantly higher phenotypic diversity than did adults at FB. Overall, there was a significant positive correlation between rockpool algal diversity and isopod phenotypic diversity.