The 1995–1998 histopathology data from NOAA's Mussel Watch Program were analyzed to: (1) document the occurrence of parasites and pathologies in sentinel bivalves during the 1995–1998 time period, (2) describe and compare the geographic distribution of these parasites and pathologies between different bivalve species and between different geographic regions, and (3) evaluate trends in parasite taxon richness. Parasite taxon richness was higher in oysters than in mytilids and dreissenids. Parasites having higher prevalences in a given host taxon routinely also had higher infection intensities. When different geographic locations were compared, the same trends occurred much more rarely. Oysters were more heavily infected based on total parasite body burden than mytilids, and the frequency of hosts with at least one parasite was higher. Excluding the numerically-dominant gregarines, however, removed the differential between oysters and mytilids, with the exception of East-coast mytilids that were more frequently parasitized than East-coast oysters. Dreissenids had lower prevalence and infection intensity for all parasites than the other host bivalve taxa. Though cestodes offer a discrepancy, most of the more common oyster parasites were unicellular, whereas most of the more common mytilid parasites were multicellular. On the average, parasite distributional patterns along a stretch of coastline were more often clinal in nature in mytilids, in that prevalence and infection intensity tended to change gradually over relatively large distance scales, and more bounded in nature in oysters, in that prevalence and infection intensity tended to change more sharply over shorter distance scales. Latitudinal trends were diametric opposites on the two northern coasts. More parasites occurred in mytilids from northern bays of the East coast, whereas fewer occurred in mytilids from northern bays of the West coast. Mytilids far exceeded oysters in the incidence of pathologies, including digestive gland and gonadal abnormalities and hemocytic infiltration. On the West coast, the vast majority of these pathologies occurred in mussels of the Mytilus edulis complex rather than M. californianus. All pathologies were more common in mytilids from the northeast coast than in West-coast mytilids. Indeed, discounting the gregarines, northeast coast mytilids combined the highest instances of pathologies with among the highest parasite body burdens of any bivalve taxon and coastal area combination in the Mussel Watch program.