1. The effect of salinity upon five Australian grapsoid crabs exhibiting differential penetration of an estuarine system has been investigated.
2. The salinity tolerances over a range of 2-70 ‰ (approx.) salinity, the acclimation times, and the osmoregulatory capabilities of these species have been determined.
3. It is concluded that for four of the five species salinity is probably a limiting factor in hyposaline conditions; whilst for only one species is salinity probably a limiting factor in hypersaline conditions. The relative abundances of two species in the estuary appears to be a function of their capabilities for osmoregulation.
4. Habitat preferences probably limit the upstream penetration of one species, and may limit the downstream penetration of a second.