Of the four families constituting the higher fresh-water basommatophora, the one on which the least amount of critical work has been done, is the family Physidae. The present investigation began as an attempt to study the genus Physa in Michigan. Some field and experimental work was carried on in 1940 and 1941 but the major portion was completed from 1949 to 1952. It became evident within a short time that, due to the chaotic state of the systematics within the group, the problem must necessarily be limited in its scope if any work of a truly critical nature was to be accomplished. Several lines of investigation have been pursued here. The life history has been ascertained through laboratory rearings supplemented by a field study of a selected natural population. An attempt to determine the ecological relationships of Physa gyrina was made through an extended survey of the waters in the vicinity of Ann Arbor and those of the Douglas Lake region. It is hoped that, by the combination of these various approaches, the biology of the species will be more adequately understood.