The life histories of carnivorous marsupials, or dasyurids, make them useful subjects for studying maternal investment, such as sex ratio and lactational investment. One group of annual breeding dasyurids are male semelparous, strongly sexually dimorphic, produce large litters that weigh two to three times the weight of the mother at weaning and show biases in siring success and sex ratio. Red-tailed phascogales Phascogale calura belong to this group and in captivity they have shown biases in siring success with body weight. The growth rates of young of this species were investigated to determine whether sex-biased maternal investment occurs. No relationship was evident between maternal weight and the sex ratio of young, indicating no sex-ratio adjustment with maternal condition. In contrast, a positive relationship was evident between maternal weight and the weight of offspring at weaning, with weaning weight being correlated with weight at maturity. Dimorphism in weight emerged during suckling, with an average dimorphic ratio of 1.5 achieved by maturity. In contrast, dimorphism in skeletal measures did not emerge until after weaning, with an average dimorphic ratio of 1.14 achieved by maturity. The sex differences in growth during suckling provide support for a male bias in maternal investment.