Field tests of Bacillus thuringiensis var. israelensis were conducted in a moderately to heavily polluted log pond in Lane Co., Oregon containing a mixed population of Culex peus and Cx. pipiens. Five dosages were applied ranging from 0.40 to 1.63 kg/ha. Reductions of 73-99% were observed after 48 hours, but with no significant differences among the dosage rates tested. One week following each test, treated areas had returned to pre-treatment levels of larvae, but with significant differences in age class distributions. Laboratory bioassays produced estimated ED50's approximately three times higher in water from the log pond compared with those conducted with distilled water.