Properties and activities of acid phosphatase in homogenates of adults of the rice weevil, Sitophilus oryzae, were studied. The optimal pH was 3.8 and Km value for p-nitrophenyl phosphate 1.75 mM. In all strains, phosphatase activity was higher in females than in males. When reared on wheat, symbiotic strains RR, LL and Sfr exhibited the same activity level while the aposymbiotic strain SS showed a higher one. There was no change in the phosphatase activity of the selected strains RR and LL maintained on sorghum, compared to those maintained on wheat; on the contrary, the activity level of Sfr and SS was higher on wheat than on sorghum. Phosphatase activities in ovaries of Sfr and SS were very similar and represented 27 and 21 per cent of the total activities of Sfr and SS females, respectively. It is suggested that the higher phosphatase activity of SS insects could be the result of the loss of their symbiotes or (and) the genetical drift of the SS strain.