The biology and immature stages of snail-killing flies (Diptera: Sciomzyidae) that are predators on land snails and slugs are presented, including the geographic distribution, habitat occurrence, life cycle, and larval feeding habits of three species of Tetanocera Duméril preying on slugs (Tetanocera clara Loew, Tetanocera plebeja Loew, and Tetanocera valida Loew), and one species attacking land snails (Tetanocera phyllophora Melander).