I assessed the occurrence of Crepidoodinium cyprinodontum on cyprinodontid and fundulid species in Maryland and Florida waters. Comparison of epibiont load across host taxa revealed Fundulus majalis as the preferred host of C. cyprinodontum. Crepidoodinium cyprinodontum infecting Fun. majalis reached a seasonal peak in epibiont load in summer in Maryland, and winter in Florida. Epibiont density decreased with increasing host (Fun. majalis) length, indicating smaller fish are more prone to colonization by C. cyprinodontum . Both numbers per fish and growth of C. cyprinodontum were influenced by light availability. Biomass based calculations of doubling time indicated that growth of C. cyprinodontum on fish gills appeared to below at optimum irradiances, suggesting refuge from predation may be a major factor in driving this dinoflagellate to colonize the opercular region of fish. Finally, I documented infections in two previously unknown host species (Fun. similis c.f. and Floridichthys carpio).