Crabs from different intertidal environments show different anaerobic metabolic responses to hypoxia and activity. I chose to compare Carcinus maenas, a species which lives in a variety of habitats and may encounter hypoxic conditions, and Pachygrapsus crassipes, which inhabits rocky zones and is not subjected to conditions of reduced oxygen availability. Rates of oxygen consumption (Vo2) and lactate production were used as indices of aerobic and anaerobic metabolic rate, respectively. Both species showed similar capacities for anaerobic energy production during activity. Activity was vigorous, of short duration, and accompanied by a rapid rise in haemolymph lactate levels. Activity in specimens of P. crassipes was primarily anaerobic. Burst running speed was constant within an ecologically relevant range of temperature. Aerobic metabolic rate, duration of activity, and maintenance of burst speed were positively correlated with temperature. Individuals of P. crassipes had a decreased metabolic rate during air exposure. Aerial Vo2 was 35% lower than the rate during submersion, but whole body lactate content was the same. Submerged crabs of both species maintained vigorous activity longer than those exposed to air, suggesting that depressed metabolism can influence activity. The two species differed in their response to hypoxia. Carcinus maenas specimens maintained Vo2 constant as water Po2 declined from 160 to 50 torr, had a reduced Vo2 as water Po2 declined from 50 to ca. 20 torr. Below this tension Carcinus maenas specimens abandoned aerobic respiration and increased lactate production. Pachygrapsus crassipes specimens neither regulated Vo2 nor increased lactate production under conditions of reduced oxygen availability. Thus, the anaerobic response to hypoxic conditions is independent of the metabolic response to activity in these species, and the metabolic responses of specimens of Carcinus maenas appear to be adaptive to the environments which that species encounters.