Weak selection at a single mutiallelic locus in a dioecious population is analysed under the assumptions of panmixia and discrete non-overlapping generations. The results hold for both autosomal and X-linked loci after several generations have elapsed. With an error of the order of s (i.e. O(s)), where s is the selection intensity, the population evolves as if it were monoecious. The equivalent monoecious fitnesses must be calculated by weighting each sex by the number of genes carried by an individual at the locus under consideration. Provided the explicit time dependence (if any) of the genotypic fitnesses in each sex is O(s2), the rate of change of the male--female allelic frequency differences is O(s2). If the change per generation of the genotypic fitnesses is smaller than second order in s (i.e. o(s2)), then to O(s2) the rate of change of the unweighted average of the male and female mean fitnesses is equal to the genic variance. Hence, as long as there is significant gene frequency change, this measure of the mean fitness of the population will increase.