Embryonic development in Physella cubensis is similar to that in other members of Physella, although trochophore or veliger-like embryonic stages previously noted in P. gyrina were lacking. Egg masses of adults collected from Shades Creek, Birmingham, Alabama were subjected to three temperature treatments (10, 20 and 30°C) within the normal range of temperatures experienced in the field, and developmental stages were documented until hatching. Hatched juveniles were raised in their respective temperature treatments to sexual maturation (onset of egg mass deposition). Temperature dramatically affected the time course of embryonic development. Juveniles (0.8 mm shell length) hatched in 36 days at 10°C, 13 days at 20°C, and 9 days at 30°C. Juveniles raised at 20 and 30°C produced egg masses 69 and 35 days after hatching and had shell lengths of 6.2 and 4.4 mm, respectively. No egg masses were produced at 10°C. Body size at first reproduction was significantly smaller in the individuals raised at 30 than at 20°C indicating that somatic growth takes precedence over reproduction at lower’temperatures. Juveniles raised at 10°C did not attain sexual maturity over 100 days and reached only 1.5 mm shell length. The pronounced effect of temperature on rates of embryogenesis, juvenile growth, and the onset of sexual maturation suggests temperature plays a significant role in regulating the population dynamics of P. cubensis. © 1990 Balaban.