Samples containing the marine ostracod Callistocythere badia were collected from upper shore rockpools on Filey Brigg, north-east England in order to examine the life cycle and genetic structure of the species. Reproduction occurred during spring/summer after which the adults died; juveniles over-wintered as A-1 instars. No males were ever found during the study. Analysis at three polymorphic loci (Gpi, Pgm and Mpi), showed significant deviations from Hardy–Weinberg equilibrium, and significant linkage equilibrium at and between all loci. The genetic data and absence of males confirm that Callistocythere is an asexual species. There was a significant positive relationship between the number of clones found within a pool and rockpool size, but not with algal habitat heterogeneity.