Fifty to eighty percent (1.5 to 2.5 ppm) of a desired 3.10 ppm (124,000 sp/ml) treatment concentration of Bacillus thuringiensis var. israelensis (Bti) was sustained over a 20 to 22 min time interval. Peak recovery occurred midway through a 35 min exposure period when treating a 312 m section of stream. Little, if any, spore residual remained in the test stream following treatment application, as noted by the rapid decline in recovered spores once application was terminated and by the near background count levels of spores remaining in stream eddies 24 and 48 hr post-treatment.