A population of Modiolus modiolus (horse mussel) inhabiting a sub-arctic environment in Logy Bay, Newfoundland, was studied for a period of two years. The main objective was to gain insight into the relationship between environmental factors (temperature and components of the seston) and the physiological response of this species. Measurements of the biochemical composition of the gonad, digestive gland and remaining tissue were made over the same period to support the physiological and environmental data. All the variables determined for the suspended particulate matter (seston), i.e. organic matter, chlorophyll a, organic carbon and nitrogen, lipid, carbohydrate and protein, and the number and volume of the particles, showed a clear seasonal pattern, with higher values during the spring and summer of each year of study. -- The highest values for energy acquisition (ingestion and absorption rates) by Modiolus modiolus coincided with the spring phytoplankton bloom occurring during April-May in Logy Bay, whereas energy expenditure (oxygen uptake and ammonia excretion rates) was greatest during the summer (July and August). The result was a clear seasonal fluctuation in the two physiological integrations, scope for growth (SFG) and net growth efficiency (K2), for which lower values were associated with a high metabolic rate, high temperature and low quality of the food supply. Conversely, higher values of SFG and K2 were associated with a low metabolic rate, low temperature and an energy-rich food supply provided by the phytoplankton bloom. -- The ash-ratio technique (Conover, 1966) was compared with other techniques for measuring absorption efficiency, and found to be a valid as well as a convenient method for use with horse mussels feeding on natural seston. -- Proximate biochemical analysis of the gonad, digestive gland and remaining tissue suggested that in Logy Bay Modiolus modiolus may compensate for the nutritive stress induced by poor food conditions for much of the year by prolonging the period over which energy reserves are accumulated, rather than by a reduction in fecundity or egg quality.