The rates of oxygen consumption, filtration and ammonia excretion by Mytilus californianus have been related to body size and to ration. The rate of oxygen consumption (VO2) by individuals while immersed, measured on the shore, resembled rates recorded for mussels starved in the laboratory. VO2 by M. californianus was relatively independent of change in temperature, with a Q 10 (13-22° C) of 1.20. In contrast, the frequency of heart beat was more completely temperature dependent [Q 10 (13-22° C)=2.10]. Filtration rate showed intermediate dependence on temperature change [Q 10 (13-22° C)=1.49] up to 22° C, but declined at 26° C. Both VO2 and filtration rate declined during starvation. The utilisation efficiency for oxygen was low (approx. 4%) between 13 and 22° C, but increased to 10% at 26° C. Three components of the "routine" rate of oxygen consumption are recognised and estimated; viz. a basal rate (0.136 ml O2 h-1 for a mussel of 1 g dry flesh weight), a "physiological cost" of feeding (which represented about 6% of the calories in the ingested ration), and a "mechanical cost" of feeding which was three times higher than the physiological cost. The ratio oxygen consumed to ammonia-nitrogen excreted was low, and it declined during starvation. These data are compared with previously published measurements on Mytilus edulis, and the two species of mussel are shown to be similar in some of their physiological characteristics, though possibly differing in their capacities to compensate for change in temperature. For M. californianus, the scope for growth was highest at 17-22° C and declined at 26° C; it is suggested that exposure to temperatures in excess of 22° C, as for example during low tides in the summer, might result in a cumulative stress on these populations of mussels by imposing a metabolic deficit which must be recovered at each subsequent high tide. The high "mechanical cost" of feeding imposes a more general constraint on the scope for activity of the species.