During the 15 months of the experiment life expectancy was reduced by removing the largest individuals in laboratory populations of guppies. The average size of maturing females was 18.7 mm in exploited groups, and 21.0 mm in the control. Males matured in exploited populations at 14.7 mm and at 16.1 in the control. The differences in both sexes represented phenotypic plasticity. Their direction agreed with predictions from the models of optimal age and size at maturity.