How nest size and temporal availability of females affects sexual selection was tested experimentally in the sand goby, Pomatoschistus minutus, a small fish with paternal brood care. The distribution of matings depended on both nest size and female arrival pattern, supporting the model for mating system evolution presented by Emlen & Oring (1977). Polygyny and the opportunity for sexual selection was greatest when nests were large and females arrived asynchronously. In these replicates a single male frequently spawned with all four females. The total number of females spawning was the same for each treatment. Male size was important for mating success in the large nest treatment, whereas female arrival pattern did not affect the size difference between mated and unmated males. This study demonstrates that sexual selection can depend on resource distribution even if all individuals can obtain similar quality resources, a situation where resource monopolization per se should not differentially affect access to mates.