A previously undescribed, very small species of Cryptodifflugia has been discovered in a calcareous wetland on the Niagara Escarpment, a significant geologic feature in southern Ontario, Canada. Cryptodifflugia leachi n. sp. is characterized by its very small size, elliptical outline in optical cross-section (contrasting it with the similarly-sized C. minuta Playfair, which is strongly narrowed in the anterior pseudostomal region) and the presence of crystalline inclusions in the anterior region of the cell. The abundance of specimens in the collection afforded study of both living and encysted specimens.