The mud whelk, Cominella glandiformis, from the Avon-Heathcote estuary in Christchurch, is infected by three species of larval digenean parasite. The prevalence of infection in C. glandiformis was found to increase with snail length for each parasite species. This introduced the possibility that gigantism could be occurring, and investigation of this formed the study basis. Differences were apparent between sexes of snail, and juvenile and adult individuals, with respect to both infection and position on the shore. For the analyses used, a combination of length and shell weight was found to be the most effective measure of snail size, but wet and dry tissue weights could also be used. In order to determine whether gigantism was occurring, a laboratory study was undertaken to investigate any growth rate differences between infected and non-infected snails. The results from this, however, were inconclusive. The question of whether gigantism could occur in C, glandiformis was therefore approached by critically evaluating three hypotheses given in explanation for the phenomenon (Minchella et al. 1985). Histological examination of infected snails was used to distinguish between each hypothesis, and this showed that gigantism was more likely to occur in C. glandiformis due to parasite adaptations, rather than transient nutritional surpluses or host adaptations to parasitism.