A study was carried out to isolate and identify actinomycetes present in activated sludge scum. Samples from two activated sludge treatment plants were analyzed. One hundred and five strains were isolated. The predominant organism in both treatment plant samples was Nocardia amarae. Fifty-one strains of N. amarae were isolated. The second most common organism (24 strains) was N. asteroides. The other isolates were identified as Rhodococcus coprophilus, R. equi, R. erythropolis, R. maris, Mycobacterium fortuitum, and strains belonging to the genera Nocardia, Rhodococcus, Oerskovia, Amycolatopsis and Micromonospora. Distribution of actinomycetes in the samples varied. This observation appeared to indicate that plant design, operational and environmental conditions might play an important role in determining the types of actinomycetes in aeration basins.