A recent evolutionary ecological model of facultative paedomorphosis predicts that body size of mature individuals should be larger than immatures of the same cohort. We investigated sex-specific differences in body size and maturation within a single cohort of branchiate (= larval and paedomorphic) mole salamanders, Ambystoma talpoideum. In addition, we also sampled the population after the breeding season, as some individuals began to undergo metamorphosis and leave the pond. The branchiate population was female-biased (62.7%), and mature (paedomorphic) females were significantly smaller than paedomorphic males or immature (larval) females. The majority of male branchiates were mature (86.6%), whereas significantly fewer females were mature (64.4%). After the reproductive season, males and females underwent metamorphosis in the same proportion in which they occurred in the branchiate population, although a significantly greater proportion of immature females metamorphosed (64.6%) compared to their frequency in the branchiate population (35.6%). There were no significant differences in body size with regard to sex or maturation among metamorphosing individuals. Our data demonstrate that maturation in branchiates is independent of body size in males and that it may negatively affect body size in females. Our findings underscore sex as a potentially important factor, and question the role of body size, in regulating this life cycle polymorphism in A. talpoideum.