The heritability of phally, a dichotomous trait defined by the functional state of the male reproductive tract, was measured in a laboratory population of the simultaneous hermaphrodite snail Bulinus truncatus by means of a breeding experiment and a selection experiment. Euphallic individuals develop a fully functional male and female tract and are capable of receiving and donating sperm. In aphallic individuals the male tract does not develop fully, preventing sperm donation. There was no evidence of a heritable component to phally in the breeding experiment, but the selection experiment demonstrated a slight heritable effect. In both experiments there was more variation in the observed proportion of euphallics than expected by chance alone and no evidence of line or family effects, implicating environmental determination of male outcrossing ability even under controlled laboratory conditions. Previous studies of populations of B. truncatus reported that the proportion of euphallics was under strong genetic control. We suggest that there may be population differences in the extent of environmental control over phally, analogous to that reported for sex determination.