Heterospecific grouping behavior of mummichogs (Fundulus heteroclitus) and banded killifish (Fundulus diaphanus) was analyzed in the laboratory and in a freshwater tidal marsh in Cremona, Maryland. Several parameters of wild, intact shoals were measured, including species composition, body length, parasite load, gender, and any physical abnormalities. Fish collected were used for laboratory analysis of shoaling preferences. When size was equal, banded killifish and mummichogs preferred conspecific shoals to heterospecific shoals, consisting of mummichogs, banded killifish, and sheepshead minnows (Cyprinodon variegates). Shoal collection from the field resulted in mixed species shoals with individuals predominantly unaffected by parasites or other physical abnormalities. Size appeared to be a sorting mechanism. A temporal shift in lengths was evident. Initial shoals caught contained significantly smaller fish compared to the final shoals caught. Results are compared with previous studies on heterospecific shoaling in killifish and new characteristics of heterospecific shoals inhabiting freshwater tidal marshes are discussed.