Cerithidea california is examined to determine the fecundity of the parasitic trematode species in its gonad. Five species of trematodes are studied (Cloacitrema michiganensis, Euhaplorchis californiensis, Parorchis acanthus, Himasthia rhigedana, and Acanthoraryphium sp.). Their fecundity is determined by direct counts of rediae and estimation of the cercarial population. Correlations are shown for trematode numbers and snail size. Incidences of infection are given with regard to site, snail size, and trematode species. Comparison is made between infection ratios of C. californica and another marsh snail, Batillaria zonalis.