How mothers allocate resources to offspring is central to understanding life history strategies. High quality mothers are predicted to favour investment in sons over daughters when to do so increases inclusive fitness. This is the case in ungulates with polygynous mating systems, where reproductive success is more variable among males than females, but information is scarce on sex allocation in less polygynous species. Here, for the weakly dimorphic roe deer, we show that as maternal capacity to invest increases, mothers increase allocation to daughters more than to sons, so that relative allocation to daughters increases markedly with increasing maternal quality. This cannot be explained by a between sex difference in growth priority, hence we conclude that this is evidence for active maternal discrimination. Further, we demonstrate that condition differences between offspring persist to adulthood. For high quality mothers of weakly polygynous species, daughters may be more valuable than sons