We analyzed growth of the sigmodontine rodent Oryzomys albigularis under laboratory conditions, fitting data to growth models, to test the null hypothesis that no differences exist between sexes. We propose a biologic criterion for growth-model selection, under the assumption that the curve should show critical stages of the organism's postnatal development and growth. Following this approach, growth of O. albigularis best fits a logistic curve. Our results show that, although males and females grow at the same rate, the final size is significantly different, being slightly dimorphic. These results, together with behavioral observations of the animals in the laboratory, support the hypothesis of a monogamic mating system for this species.