The purpose of this study is to trace the life history of Sphaerium partumeum, and to discover in what ways it may be modified by the environmental conditions of a temporary pond. The project is divided into three interrelated parts: growth studies carried on in both field and laboratory, investigation of reproductive processes, and population analysis based on two years of collecting in a single habitat. The growth of Sphaerium (Musculium) partumeum (Say) is indeterminate. Animals attain almost full size in a period of seven to ten weeks, and thereafter grow very slowly until they die. In this study the terminal size reached by field specimens was slightly greater than that for animals reared in the laboratory. ... Sphaerium partumeum is capable of self-fertilization and can be raised in isolation without loss in vitality of the stock. It seems likely that at least part of the time the species reproduces by the same means in the natural habitat. ... The life span of Sphaerium partumeum in the field is normally thirteen or fourteen months. It extends, in this particular pond, from June or July of one year to mid-summer of the following year. ... The fact that this pattern of life is impressed on these animals by their environment is proved by comparison with the laboratory-born specimens, which grew and produced young at any time of year, without showing any seasonal correlation.