The study of the effect of activity on physiological rates (ammonia excretion, oxygen consumption and filtration) was carried out at ambient temperature and food level on two species of clams, Tapes philippinarum and Tapes decussatus, in the period 9 August 1992-6 October 1992. Clams in the experimental vessels exhibited various states of activity which were rated by identifying and scoring (0-4) the different parameters, including shell gape, siphon extension and foot protrusion, The scores assigned were well correlated with physiological rates (excretion, oxygen consumption and filtration) although the values given were non-linear. No significant differences between the two species in oxygen consumption or filtration rate were detected, However, the excretion rate of T. decussatus was significantly higher than that of T. philippinarum (P < 0.05), In addition, the absorption efficiency of both species was positively correlated with particulate organic matter level in the seawater (P < 0.001).