Competing theories of sex allocation in mammals may best be reconciled in the light of data from diverse species. The tammar wallaby (Macropus eugenii) is potentially a particularly interesting study animal because females wean only one young per year, and exhibit extreme synchronicity in the annual onset of breeding. By contrast, reproduction in the closely related parma wallaby (M. parma) is almost asynchronous. These two Australian species are found sympatrically only on Kawau Island, New Zealand, where they were introduced in about 1870. We sampled wallabies on Kawau Island in April of 1996, when both species were breeding. Although the sex ratios in both species were not significantly different from unity, offspring of M. eugenii were very significantly more likely to be male with increasing maternal weight (logistic regression χ2 = 16.8, P < 0.0001), and the fewer M. parma data showed a non-significant trend in the same direction (χ2 = 1.9, P = 0.16). These data, at least for M. eugenii, are consistent with the Trivers-Willard hypothesis, and warrant further investigation in wild and captive populations under different measured or manipulated ecological conditions. We suggest an approach utilising the characteristics of M. eugenii which might help determine whether the sex bias is determined close to conception, or is effected later in the reproductive cycle by differential survival of the sexes.