Susceptibility of southern buffalo gnat larvae to Bacillus thuringiensis var. israelensis (B.t.i.) was studied during 1986-88. Tests were conducted using Vectobac 12AS in a mini-gutter aquatic bioassay system. Mini-gutter tests using a 4.5 ppm B.t.i. concentration and a 10 min exposure period confirmed a positive correlation between water temperature and B.t.i. effectiveness. Significantly lower larval mortality occurred as water temperature decreased below 9 degrees C, indicating that proper timing of B.t.i. application is essential to maximize larval control. Even with this temperature limitation, larval control using B.t.i. should provide an economically effective means of preventing outbreaks of the southern buffalo gnat.