Shells of larvae of fresh-water bivalves (known as glochidia) of the family Unionidae, belonging to Unio and Anodonta, have been found by the present writer in Holocene and Pleistocene fresh-water deposits of Poland. Fossil glochidia were never described before. Their size is the most important generic character distinguishing glochidia. Glochidia, which happen to be preserved on the apexes of young shells or on the fragments of older shells, in the form of what is known as prodissoconchae, allow one to determine the generic assignment of these otherwise indeterminable fossil specimens of Unionidae. Since the sediments, in which they are found, correspond to the spring-summer period, they also serve as indexes of temperature of a sedimentation environment.