In order to develop standard conditions for rearing the gray short-tailed opossum, Monodelphis domestica, as a potentially useful experimental laboratory animal, the effects of four different diets on growth and reproduction were assessed. One diet was a meat-based diet prepared in the laboratory. The other three diets were commercially produced fox foods designated Reproduction diet, Lactation diet, and Growing and Furring diet. All pairs of M. domestica fed the Reproduction diet produced at least one litter, but only two-thirds or fewer of the pairs fed any of the other three diets reproduced. There were no significant differences in the number of young born per litter or the number of young weaned per litter among the diets. Weight at weaning was significantly lower for individuals on the meat-based diet compared to those on the fox food diets. Young on the meat-based diet suffered 50% mortality within 6 weeks after weaning, whereas none of the animals fed the fox food diets died within the same 6-week period. Age-weight data were described using the Bertalanffy growth function. In terms of growth and overall reproductive performance, the fox food diets were clearly superior to the meat-based diet, and the Reproduction diet was judged to be the best of the fox food diets tested. Growth curves, from birth to 550 days of age, of individuals fed the Reproduction diet were developed and can be used as standards for the species under laboratory conditions. The maximal weights attained by animals fed the fox food diets were similar to the weights of the wild-caught founders of the laboratory population, indicating that the fox food diets provide adequate nutrition for normal growth. An additional observation was that females housed singly past the normal age of sexual maturity attained significantly lower adult weights than did females that were paired with males at 6 months of age.