The intertidal isopod, Ligia oceanica, inhabits humid micro-habitats at the high water mark of rocky shores, where it experiences elevated temperatures in the summer due to solar radiation. Temperature tolerances vary seasonally with critical maximum temperatures falling from 37.4°C in the summer, to 34. 9°C in the winter. In general, whole-animal rates of oxygen uptake and protein synthesis were higher in winter compared with summer animals. In both seasons, rates of oxygen uptake were more sensitive to temperature change than rates of protein synthesis, due to a compensatory increase in RNA levels in the winter. Patterns of heat-shock protein induction varied between seasons, with acclimatised Ligia in the summer showing higher constitutive and de novo heat-shock protein synthesis levels than winter Ligia. In the summer, animals synthesised heat-shock protein 60 at two induction temperatures of 27°C and 31 °C. Animals collected in the winter, however, synthesised hsp60 at the lower induction temperature of 25°C, and continued to express hsp60 at 27 and 29°C. Acclimated Ligia showed delayed heat-shock protein induction and an overall attenuated heat-shock response relative to acclimatised animals, regardless of season. Estimates of the metabolic costs of general protein synthesis revealed that in the winter, costs were relatively expensive and accounted for 70.0% of oxygen uptake rates at 40.9 μmol 02.mg protein-1. In the summer, metabolic costs were 10-times lower at 3.9 μmol 02.mg protein-1, accounting for 17.3% of oxygen uptake. Temperature acclimation had no effect on metabolic costs, but winter values, expressed as the proportion of oxygen uptake, showed remarkable similarity to those found in an Antarctic isopod. Metabolic costs of heat shock protein synthesis were 2.4-times higher in the w inter than in the summer, showing that energy requirements for survival are higher at a time when energy stores may be restricted.