We estimated age and size at onset of functional outcross- and self-fertility in three isofemale lines of Physa heterostropha carrying the recessive, non-allelic genes, alb1 and alb2. `Experimental' snails of these lines were reared from 2 weeks of age to adulthood and periodically introduced to mature, complementing `challenge' snails. The three lines differed significantly in the size at which they matured, but not the age, under our culture conditions. Male reproductive maturity was reached at a mean of 5.7 weeks and female reproductive maturity at 7.3 weeks (n = 50). These ages correspond to mean shell lengths of 5.3-6.2 mm as male and 6.9-7.6 mm as female. Over all three lines, first production of viable offspring by self-fertilization occurred at a mean age of 22 weeks (n = 113). Again a significant line effect on size at reproduction was detected, with mean size at onset of self-fertilization ranging from 8.1 mm to 8.7 mm. Autosterility in the three lines ranged from negligible to 44%. Among autofertile snails, we identified cases of outcross male-sterility, outcross female-sterility, and outcross double-sterility. Of 46 individuals demonstrating male function before female, two passed through brief periods of self-fertilization before outcrossing as females. We also identified three individuals maturing simultaneously in both capacities and one individual that matured as female before male. So, although the situation is complex, we suggest that `simultaneous hermaphroditism' provides the best description of the reproductive biology of Physa. The great diversity of reproductive allocation described here implies considerable potential for life-history evolution in these snails.