In sexually reproducing species, resources may theoretically be distributed with bias to the production of male or female offspring in response to the condition of the mother, commonly recognized as sex allocation. Using a recently characterized sex-specific molecular marker, we tested for maternal sex allocation (i.e. maternal primary sex ratio bias and sex-specific offspring investment) in captive laboratory-bred western mosquitofish (Gambusia affinis) at early stages of offspring development. We found no statistical evidence to support sex allocation in G. affinis, based on maternal condition. In addition, we found little evidence for correlations between maternal condition and investment in the condition (mass) of individual offspring (of one sex or the other), although we did find that larger mothers tended to have higher fecundity.