The idea that offspring sex ratio is affected by parental capacity for reproductive investment was first developed by Trivers and Willard1 and later elaborated by others2-6. According to this hypothesis, if the amount of parental investment7 differentially affects the expected reproductive success of offspring depending on offspring gender, then parents capable of exceptionally large investment should bias reproductive investment toward the sex affected most. For most polygynous mammals, male reproductive success is expected to be most enhanced by exceptionally large investment1,8, but observational field studies and laboratory experiments have either supported9-13 or contradicted 3,14-16 this conditional sex-allocation hypothesis. We have performed the first experimental field study specifically to examine this hypothesis, and find that the female common opossum (Didelphis marsupialis), whose capacity for parental investment was artificially enhanced by dietary supplementation, produces male-biased sex ratios, in contrast to the unbiased ratios produced by control females. © 1986 Nature Publishing Group.