High concentration of heavy metals is a toxic factor for most microorganisms. As a result, such metals give rise to severe disruption in wastewater treatment operations. It has been noticed, however, that biological systems may adapt to heavy metals when their concentration is limited. In fact, such systems can even produce some degree of metal elimination provided that an optimal exposure rate is guaranteed. On the other hand, irreversible damage may occur when metal concentration is sufficiently high. It is the aim of the present study to examine the inhibiting effects due to the presence of cadmium by the activated sludge treatment method. To this end, different Cd concentrations were tested (ranging from 0.25 to 14 mg 1(-1)) in a Sequential Biological Reactor, using synthetic wastewater without recirculation. The feed water provided 325 mg 1(-1) COD and 30 mg 1(-1) NH4+-N. The process was screened by checking elimination percentages for COD and ammonia, as well as the evolution of ciliate protozoa. In addition, research was carried out on Cd assimilation by the activated sludge system by means of measuring in the mixed liquor and in the effluent. Results showed a marked decrease in purification efficiency when Cd-concentration rose over 14 mg 1(-1), in which case COD elimination dropped from 98% to 41%, while ammonia elimination went down from 98% to 32%. A Cd concentration above 10 mg 1(-1) would result in a reduction of more then 80% of protozoa in the activated sludge.