Asexually reproductive stages equivalent to the cercaria and the metacercaria, with oral and ventral suckers, bifurcate caeca, and forked tails were found between the mantle and shell of subtidal snails collected in 1977 and 1978. The parasites were found in 77% of 336 Margarites costalis (Gould), 59% of 27 Margarites pupillus (Gould), and 59% of 37 Margarites olivaceous (Brown). With heavy infections, the extrapallial fluid of the snail is increased and the gonad and digestive gland may be destroyed. Host amoebocytes increase in the extrapallial fluid and the larvae may become encapsulated by melanin and nacreous secretions. Unusual crystals within the intestine of the parasites may be by-products of the ingested medium.