This thesis investigates aspects of growth and reproduction in the marine bryozoan Celleporella hyalina (L. ). It was found that C. hyalina could live and grow in the laboratory on a range of algal diets. Chiorophyte algae proved unsuitable, but Rhodomonas baltica proved outstanding as a foodstuff. Astogeny of Rhodomonas fed colonies was the same as that reported for naturally occurring colonies, and resulted in the production of viable larvae. It was further noted that female zooids were occasionally produced in the basal layer. C. hyalina colonies were able to grow and attain sexual maturity in cell concentrations ranging from 10 to 300 cells µl1 Growth was greatest at around 100 cells µl, ' and was suppressed at low and extremely high cell concentrations. Food supply had a highly significant effect on growth parameters, but sexual parameters were largely unaffected. Cell ingestion rate increased as a function of cell concentration, whereas feeding episode length declined with increasing food supply. Feeding behaviour in C. hyalina is described. Highly significant genotype-environment interactions were found at all levels for both somatic and sexual parameters in 26 C. hyalina clones grown in four temperature/food supply combinations (macroenvironments). The number of autozooids per unit area was found to be strongly temperature dependent. The number of ovicells was consistently greater than the number of embryos produced. Male investment was favoured under conditions of low resource. Ranking of clonal performance varied considerably, both between macroenvironments, and according to the measure of fitness used. Significant levels of intraclonal variation occurred within replicates of a single genotype grown in four macroenvironments. Variance was more prominent in somatic rather than sexual parameters. The presence of one statistical outlyer suggested that somatic mutations may sometimes occur. C. hyalina colonies grown in isolation produced embryos sometimes as early as five weeks, but the number was low, and a high incidence of abortion was observed. When removed from isolation, the number of embryos increased significantly. There was no evidence of settlement from larvae produced in isolation.