ABSTRACT: Monthly replicate samples of 4 species of common rockpool algae (Ceramium nodulosum, Chondrus crispus, Cladophora rupestris, and Corallinaofficinalis) were collected in order to investigate the changes in ostracod species abundance,richness and diversity both between the algae species and over a 12 mo period. In order to determine if algal structural complexity had an effect on ostracod abundance and diversity, structural complexity was assessed by counting the number of branchescm^-1 frond length. There were significant differences in complexity between the 4 algal species. Ceramium had significantly more branches cm^-1 than the remaining 3 algal species, and Cladophora had significantly morebranches than Chondrus and Corallina. The abundance and diversity of ostracod assemblages demonstrated pronounced seasonal variation, both within and between the 4 algal species. Ceramium supported a higher abundance, greater speciesrichness and greater diversity of ostracods throughout the year than did Chondrus, which had a less complex structure. The Chondrus samples formed a separate cluster during classification and ordination, reflecting the relative paucity ofspecies and low densities found on that algal species throughout the year. A total of 18 species of ostracod were found on the algae, but only 8 occurred at densities sufficient for determination of their life-cycles. Cythere lutea reproduced inearly spring and demonstrated positive covariation in population density with Hirschmannia viridis. H. viridis and Heterocythereis albomaculata demonstrated positive covariation in population density and had similar life-cycles. All 3species were found at low densities during the winter months, possibly as the result of seasonal migration to sublittoral areas. The remaining 5 species all demonstrated positive covariation in population densities, but reproduced at different times ofthe year. Semicytherura nigrescens and Xestoleberis aurantia underwent reproduction during summer/autumn. Both Paradoxostoma variabile and P. ensiforme appeared to be reproductively active for most of the year, but P.hibernicum appeared to be an annual. All 8 species occurred in significantly higher densities on Ceramium than on Chondrus, and during the peak reproductive period as compared to other times. The seasonal changes in ostracod densitywithin the rockpool can be primarily attributed to the reproductive cycles of the component species within the assemblage, and secondarily to the supposed seasonal migration of certain species between intertidal and sublittoral regions.