Recent studies have indicated that mating success of large males may improve under increasing levels of mating competition. This outcome is explained 1) if male mating competition is overriding female preferences for male traits that are unrelated to, or negatively correlated with, male size and dominance and, in so doing, dictates the distribution of matings or 2) if females alter their preferences with respect to large males when male-male competition is intense. Under both hypotheses, one could expect large, dominant males to be more successful under intense competition than under weak competition. However, only the first explanation predicts that male mating success under intense competition should be determined by dominance; traits that are unrelated to male dominance should be uncorrelated to mating success. In contrast, if females change their preferences (explanation 2), males with traits beneficial to females independent of the competitive environment can maintain a high mating success under all levels of male-male competition. We tested these alternatives using a small marine fish, the sand goby, Pomatoschistus minutus. The mating success of large males increased under conditions allowing intense male competition, whereas females showed a preference for good nest building independent of the level of competition. These findings suggest that females are in control of their choice by altering their preference for male size in response to the intensity of male-male competition rather than female preference being overshadowed by male dominance. This plasticity of preferences implies that the strength of sexual selection is not constant at the population level.