Test data from 2 field trials showed that extensive growth of Potamogeton crispus and P. pectinatus had varying effects on the movement of Bacillus thuringiensis var. israelensis (Bti) through a 312 m section of test stream. During on 35 min exposure time, 24 to 90% (0.8 to 2.8 ppm) of a 3.10 ppm Bti treatment concentration was recovered over an 18 to 28 min period. However, during a second field trial, suspected stream channelization resulted in a significant increase in recovered spores ranging from 62,000 to 205,000 spores/ml (1.9 to 6.4 ppm, i.e., 61 to 206%).