Ulcerative lesions and mass mortalities of Atlantic estuarine fish,particularly menhaden (Brevoortia tyrannus), have been associated withexposure to Pfiesteria-like dinoflagellates and their toxins. We willdiscuss the pathology of fish collected from the Chicamacomico River,Maryland. In the majority of menhaden sampled we observed solitaryulcerative lesions on the trunk or around the vent. One striped bass(Morone saxatilis) had an area of reddening around the base of the dorsalfin. Bluegill (Lepomis macrochirus), channel catfish (Ictaluruspunctatus), yellow perch (Perca flavescens) and carp (Cyprinus carpio)were externally non-remarkable. Histologically, ulcerative menhadenlesions demonstrated a marked chronic inflammatory infiltrate in largeareas of exposed necrotic muscle. The ulcers contained granulomata withfungal hyphae in the necrotic tissue. Gram negative rod-shaped bacteriawere also observed in the lesions, a common finding in ulcers of aquaticorganisms. Our data suggest that "typical" ulcerative lesions observed onfish from areas of Pfiesteria-like dinoflagellate blooms are reflective ofdermatosis which may be related to a variety of individual or combinedenvironmental stressors. Exposure to dinoflagellate toxin(s) potentiallyrepresents one such stressor, and the role of Pfiesteria-likedinoflagellate toxin in fish primary lesion development is currently underinvestigation. We will also discuss the presentation of differentulcerative lesions observed in other Chesapeake Bay fish pathologystudies. These studies have been supported in part by the U.S. ArmyGarrison Aberdeen Proving Ground, Installation Restoration Program, and agrant from the U.S. Environmental Protection Agency #CR826913-01-0.