In Caenorhabditis elegans the two sexes, hermaphrodites and males, are thought to be irreversibly determined at fertilization by the ratio of X chromosomes to sets of autosomes: XX embryos develop as hermaphrodites and XO embryos as males. We show instead that both sex and genotype of C. elegans can be altered postembryonically and that this flexibility requires sexual reproduction. When grown in specific bacterial metabolites, some XX larvae generated by mating males and hermaphrodites develop as males and lose one X chromosome. However, XX larvae produced by hermaphrodite self-fertilization show no such changes. We propose that sexual reproduction increases developmental flexibility of progeny, allowing for better adaptation to changing environments.