In colonies of the cheilostome bryozoan Celleporella hyalina (L.), water flow regime has a significant effect on colony growth rate and, indirectly, on the number of reproductive zooids produced. Higher growth rates occur under conditions of higher water flow. Sex ratio and reproductive output are not, however, significantly affected. Colonies of different genotypes show significant differences in their reproductive versus somatic investment, and in their allocation of resources to male and female functions. There is therefore genetically based variation in life history strategy within the population of colonies. This variation may reflect limitation of normalizing selection imposed by microenvironmental variability.