This study of 3 northern US Gulf Coast estuaries (Pensacola and Escambia Bays in Northwest Florida; Mobile Bay, Alabama; Pascagoula Harbor, Mississippi Sound, Mississippi) investigates: (1) frequencies of known or new diseases, including neoplasms, in oysters and fishes at sites among the estuaries; (2) general relations between disease frequency and human activity in influencing disease prevalences in fish and shellfish populations in coastal regions characterized by varying degrees of human impact. One offshore control station was collected quarterly (1979-1980) for fish-disease prevalence in comparison with the estuarine stations. Monthly samples of fishes and oysters were collected for disease analyses and diagnoses from August 1978 through August 1980. Sampling stations near heavily industrialized sites in Pascagoula Harbor yielded a higher frequency of diseased fishes and oysters compared to sampling stations in the other estuaries. Even though there were significant differences among disease prevalences in the 3 estuaries, the total prevalence of diseases in fishes and oysters suggested no grossly evident epizootics of pollutant-related disease at the time of study. However, further in-depth study of certain oyster and fish disorders encountered is recommended. Though far from pristine, the estuaries studied were not severely unhealthy environments as evidenced by scattered and relatively low disease prevalences. However, because the Gulf Coastal Plain will probably be the fastest growing region in the USA in the next decade, this study assists in providing base-line data for studies that should be made periodically on the health status of coastal biota in relation to the expected increase in human impact.