Cooney, SeanI assessed the occurrence of <em>Crepidoodinium cyprinodontum</em> on cyprinodontid and fundulid species in Maryland and Florida waters. Comparison of epibiont load across host taxa revealed <em>Fundulus majalis</em> as the preferred host of <em>C. cyprinodontum</em>. <em>Crepidoodinium cyprinodontum</em> infecting <em>Fun. majalis</em> reached a seasonal peak in epibiont load in summer in Maryland, and winter in Florida. Epibiont density decreased with increasing host (<em>Fun. majalis</em>) length, indicating smaller fish are more prone to colonization by <em>C. cyprinodontum</em>. Both numbers per fish and growth of <em>C. cyprinodontum</em> were influenced by light availability. Biomass based calculations of doubling time indicated that growth of <em>C. cyprinodontum</em> on fish gills appeared to be low 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 (<em>Fun. similis</em> c.f. and <em>Floridichthys carpio</em>)en-USThesisBiology, Ecology