Positive assortative mating is a widespread phenomenon in the animal kingdom. Several hypotheses have been proposed to explain this reproductive pattern in natural populations, but growing evidence suggests that assortative mating most often results from sexual mating preferences. Recently, conditiondependent mate choice in the face of costly competition for mates has been proposed to explain assortative mating in natural populations. Variation in competitive ability may generate variation in both the strength and the direction of mate preference, resulting in assortative mating with respect to individual quality if low-quality competitors are less choosy, or if high-quality males and females pair up first, leaving low-quality individuals to form pairs between themselves. We investigated individuals' preferences for mates on the basis of differential competitive abilities and size-dependent costs in mateguarding behaviour in the amphipod Gammarus pulex. Our results show that the presence of an unpaired male close to a pair can induce stress that can lead to pair separation. However, no direct take-over was observed. In our series of male mate choice experiments, under low, high or simulated levels of competition, small males made similar choices. Our results suggest that the frequency of take-overs, and more generally interference competition, can be considered insufficient to induce a reduced preference for large females in this population, and is thus unlikely to explain size-assortative mating.