Males that adopt alternative mating tactics within a conditional strategy often undergo costly morphological changes when switching to the next phenotype during ontogeny. Whether costs of changing to a subsequent reproductive phenotype are outweighed by a higher mating probability may depend on the frequencies of different phenotypes in a group of competitors. Benefits and costs associated with different phenotype frequencies depend on interactions within and between alternative phenotypes, but the underlying behavioural mechanisms have rarely been studied. Herein, we used the rock shrimp Rhynchocinetes typus as a model: ontogenetic male stages of this species differ in morphological and behavioural traits that indicate alternative reproductive phenotypes. The small, subordinate, male stage (typus) develops via several intermediate stages (intermedius) to the dominant male stage (robustus): in competitive interactions the typus males usually employ the sneaking tactic, while the robustus males invariably employ the monopolizing fighter tactic. In laboratory experiments, we manipulated phenotype frequencies to examine whether there are frequency-dependent effects on searching behaviour, aggressiveness and mating probability. With increasing frequency of robustus males, the rate of aggressive interactions among them increased. Furthermore, robustus males increased walking velocity when more than one robustus male was present. In contrast, typus males did not adjust their searching or aggressive behaviour. The increase of aggressive interactions among robustus males provided more opportunities for typus males to seize a temporarily unguarded female. While typus males exploit fights among robustus males that produce mating opportunities for them, robustus males benefit from typus males, which reveal the presence of receptive females. We suggest that each phenotype benefits from the presence of the other phenotype and suffers costly interference among individuals of the same phenotype. Whether frequency-dependent effects on the mating probability of subordinates also affect their ontogenetic switchpoint should be examined in future studies.