Vibrio fischeri bacteria, used as a biological target in either acute or chronic toxicity tests, display a low sensitivity to Cr(VI). This phenomenon could be due to the capacity of these bacteria to reduce Cr(VI) into Cr(III). This reducing capacity was found to depend on culture medium composition, pH value, incubation time and the presence of a carbon source. It also depends on the nature of the carbon source, glucose being more efficient than glycerol. This is probably related to differences in bacterial metabolism when given either glucose or glycerol. The thermostable Cr(VI)-reducing activity found in the supernatants of V. fischeri cultures grown on glucose suggests that, under these conditions, the bacteria release non-proteic reducing substances which have not been identified yet.