Bacillus thuringiensis was recovered at a high frequency from activated-sludge system environments in an urban sewage-digestive plant. All of the test materials, sampled at several digesting steps, contained the organism. Of 515 colonies belonging to the B. cereus/B, thuringiensis group, 45 (8.7%) were assigned to B. thuringiensis. The highest density of this bacterium was 1.6 X 10(3) cfu/ml in a scum sample of the first aeration basin. Among the 45 isolates, 7 were assigned to the known H serovars, Two isolates of the serovar kenyae isolates exhibited Lepidoptera-specific toxicity. Diptera-specific toxicity was shown by an isolate of serovar israelensis and a serologically undefined isolate. Lectin activity was associated with 12 isolates.