Biological and morphological data are presented on 10 Nearctic, 4 Neotropical, and 2 Palearctic species of Sepedon (Diptera: Sciomyzidae), the larvae of which kill and consume fresh-water, pulmonate snails. These data include: seasonal aspects, geographic distribution, habitats, and natural enemies; activities of adults such as feeding, mating, and oviposition; incubation and hatching of eggs; food snails and feeding behavior, growth and development, and aquatic adaptations of the larvae; and puparium formation and emergence. This information is based on laboratory rearings of all species through complete life cycles, augmented with observations in the field. The functional morphology of sciomyzid larvae in general is summarized as a background for more detailed descriptions of larvae of Sepedon. Eggs, the 3 larval instars, and puparia are described, figured, and compared, with particular emphasis on diagnostic features. Keys are presented for eggs, mature larvae, and puparia. Illustrations include photographs of egg masses, drawings of puparia and of diagnostic larval features, and maps showing the known distribution of all species included. Sepedon is one of the most widespread and best-known genera of Sciomyzidae in the world. Adult flies tend to stay in or near marshes, swamps, and pond margins used as larval breeding sites. They remain active in all except near-freezing weather. Female flies are known to lay only a few eggs at a time, but to oviposit at frequent intervals for several weeks. Larvae kill snails quickly, remain in the shells only long enough to feed to repletion, and attack other snails when hungry again. They consume aquatic snails of all common pulmonate families with little selectivity, and each larva commonly destroys more than a dozen individuals. Preliminary experiments in the use of aquatic, predatory sciomyzid larvae for the biological control of undesirable snails are cited and summarized. The relevance of this research is discussed in the hope of stimulating other studies similarly focused on a group of closely related organisms, and designed to illuminate the comparative ecology and morphology of all stages in the life cycle.