Eastern oysters (Crassostrea virginica) inhabit highly variable environments and are exposed to large seasonal shifts in temperature. Prevalence and intensity of oyster diseases, particularly Perkinsus marinus (Dermo), increase during thermally stressful periods, thus posing additional stress on the oyster host. Heat shock proteins (hsps) are important in protecting organisms from thermal and overall environmental stress. Additionally, hsps may play protective roles for both the host and parasite during infection. The interactive effects of temperature and disease on heat shock protein expression in oysters, however, are unknown. In this study, using slot and western blotting assays, seasonal and intraspecific variation in heat shock protein 70 (hsp70) expression was compared among stocks of C. virginica known to be resistant or susceptible to Dermo at two sites in the Chesapeake Bay. Mortalities, shell heights, condition, and P. marinus infections were also compared among stocks to examine relationships between hsp70 and these variables. Hsp70 was analyzed at 4 seasonal samplings (fall, winter, spring, and summer months), while all other variables were measured bimonthly. Patterns and amounts of hsp70 expression varied significantly across different seasons, but did not correspond with seasonal temperature. Total amounts of hsp70 were significantly highest in the fall. Seasonal variation in specific isoforms of hsp 70 (69 kDa and 72 kDa) was observed. Highest amounts of each were expressed in the spring and fall, respectively, and they were inversely proportional to each other. Differential expression was observed during the winter and spring, with several individuals expressing only hsp72 in the winter and only hsp69 in the spring. Although hsp72 changed concurrently with seasonal changes in infection, both hsp72 and hsp69 did not vary significantly between stocks or with levels of P. marinus infection. This study reveals that measuring total levels of hsp70 do not sufficiently describe the effect of seasonal temperatures on hsp70 expression. Stock mortalities were consistent with the patterns of disease resistance exhibited by their stock parentage, implying existence of a strong genetic component to resistance to Dermo disease. Differences in shell heights, condition index, and P. marinus infection differences showed significant associations among stock, site, and time. Variation in hsp70 did not reflect differences in infection among oyster stocks, indicating that hsp70 may not be a useful indicator to distinguish the effects of pathogenic stress between resistant and susceptible oyster stocks. Differences in expression between hsp69 and hsp72 suggest that seasonal patterns of specific hsp70 isoforms must be understood to determine the role of hsp70 proteins in stress and disease resistance in oysters.