Aerobic cellulolytic bacteria were ranked according to ability to degrade cellulose azure and to clear cellulose agar. Cellulomonas uda NRRL B404 and Cellulomonas sp. NRC 2406 showed greater clearing of cellulose agar than other isolates, but differences in cellulose azure decomposition were not statistically significant. Cellulomonas uda NRRL B404 was the slowest growing isolate, although growth rates of isolates did not differ statistically. Cellulomonas uda NRRL B404, Cellulomonas sp. NRC 2406, Cellulomonas fimi NRRL B402, Cellulomonas flavigena NRC 2403, and Cellvibrio gilvus ATCC 13127 were selected as the best candidates for larger scale experiments. Persistence of Cellulomonas uda, Cellulomonas sp. NRC 2406, and Cellulomonas sp. CS1-1 in stream-bed sediments was studied in flowing-water microcosms, using fluorescent antibodies and epifluorescence microscopic counts to assess densities of target cells. Isolate densities declined from postinoculation maxima, but organisms were detected 2-4 weeks later in three different experiments.