Small (3.24-31.70 mg dry weight) juvenile clams of a native British species (palourde,Tapes decussatus L., 3 size groups) and an introduced species (Manila clam, Tapes philippinarum Adams & Reeve, 4 size groups) were maintained either unfed or with low algae rations at 3°C, 6°C and 9°C for up to 11 wk. The various size groups responded in a similar way within each experimental treatment. At 9°C, all the juveniles survived for 11 wk. Unfed clams utilised biochemical reserves and lost weight and condition during this period. Increases in weight, reserves and condition in fed juveniles were greater in Manila clams than in palourdes. At 6°C, all Manila clam juveniles survived and fed animals showed a small increase in growth and condition. All palourdes and unfed Manila clams lost weight and condition and the mortality of palourdes was 24-46% after 11 wk. At 3°C, the majority (>88%) of juveniles of both species survived short (3 wk) periods without food. All juveniles utilised reserves and lost weight and condition, with a greater decline in palourdes. High mortalities (>50%) of all palourdes and unfed Manila clams occurred after 3 wk. At all temperatures, a greater proportion of carbohydrate than lipid was used as an energy reserve. Total lipid content and the proportion of polyunsaturated fatty acids (PUFAs) in the neutral lipids increased in treatments in which juveniles grew and decreased where a loss of organic weight was observed. In both clam species the proportion of (n-6) fatty acids, principally 20:4 (n-6), in the phospholipids appeared to indicate a response to stress; juveniles in treatments in which total organic weight decreased contained proportionally more than juveniles which grew. Food cell consumption rates were lower at lower temperatures and at each temperature were higher in Manila clams than in palourdes. Respiration rates increased with temperature only in fed Manila clams. In unfed palourdes respiration rates were higher at 3°C than at 6°C and 9°C. Results are discussed with respect to recruitment and distribution of these clams in relation to winter survival of small juveniles.