Microhabitat partitioning is a widespread mechanism that reduces competition and thus facilitates the coexistence of species. The extent to which microhabitat partitioning occurs depends on a variety of environmental parameters and biotic interactions. In the present study, we manipulated factors (animal density, presence of heterospecifics, and presence of predator cues) that potentially influence the differential use of microhabitats by 2 hemi-sessile congeners that coexist on small spatial scales at very high densities: the amphipod crustaceans Jassa marmorata and J. herdmani. In both species, the presence of heterospecifics had a clear effect on which part of an offered macroalga was preferably colonized, suggesting that the extent of microhabitat partitioning depends on the presence/absence of heterospecifics. Furthermore, 'predatory fish cues' in the seawater induced an avoidance behaviour, which should reduce the extent of habitat partitioning and inevitably increase competition between the species. The results clearly show some flexibility of habitat selection in, and thus habitat segregation between, the studied species, allowing for a trade-off between interspecific competition and predation pressure.