Population density is likely to determine the form of competition in which males are engaged for access to females. At low density, scramble competition should be of paramount importance because of the low probability of encounter between males and females. Consequently, sexual selection should favour characters that allow rapid detection of females. Conversely, at high population density, sexual selection should favour attributes that raise the fighting ability of males because of the more frequent contacts between males. These general predictions were tested in this study for the precopulatory mate-guarding isopod, Asellus aquaticus. In this species, male-biased sexual dimorphisms are reported for body size and antennae length and are, respectively, ascribed to contest and scramble competition over females. Therefore, the relative strength of sexual selection on male body size and antennae length at two different densities was experimentally assessed. Multivariate analyses indicated that density affected morphological correlates of the mating success of males, with body size being the main determinant of pairing success in males at a high density, whereas only antennae size significantly affected access to females at a low density. The antennae length of males was manipulated to examine how antennae length affects the ability of males to detect females in three experimental conditions varying in the probability of random contacts with receptive females. An advantage of having long antennae was only observed when there was an intermediate difficulty in finding females. Our results are discussed in relation to the influence of density on selective regimes in A. aquaticus.