Various aspects of the ecophysiology of the estuarine mud crab Hemigrapsus crenulatus (Milne Edwards, 1837) have been studied. Field studies have shown that the population of crabs in the Avon-Heathcote Estuary experience a seasonal variation in salinity and temperature, such that the environment is more dilute and colder during the winter than in the summer. Experiments using crabs direct from the field showed that these animals undergo a seasonal acclimatization which results in winter crabs having a greater tolerance to dilute (1%) sea water than summer crabs. This change in salinity tolerance is accompanied by winter animals having a slower rate of sodium loss, a lower apparent water permeability and a 'stronger' regulation of blood osmotic pressure in 100% sea water, than summer collected animals. Laboratory studies showed that, compared with crabs acclimated to full strength sea water, animals acclimated (for 5 weeks) to low salinities were more tolerant of 1% sea water, had a slower sodium loss rate and an osmoregulation curve that was shifted towards the isosmotic line. However, no long-term effect of salinity acclimation on apparent water permeability was found. It is suggested that salinity does playa role in the seasonal acclimatization, but some other factor(s) also appears to be involved.