The bean clam, Donax gouldii, is an intermediate host of the monorchid trematode Postmonorchis donacis. Bean clams were collected from nine locations in San Diego County, CA, and siphons and mantle edges examined. Two hypotheses were tested: (1) parasitism increases with valve length, and (2) female clams have more parasites than males. A positive relationship was found between clam length and parasitism at all locations; there was no significant difference (a 5 0.05) in male and female parasitism rates. Spatial variation on a kilometer scale was observed in trematode infestation rate and intensity.