Bacterial growth and trimethylamine oxide (TMAO) reduction were measured by following the change in conductance of the growth medium. The method was used as a reliable taxonomic test for the ability of bacteria to reduce TMAO. Conductance measurements were also applied to assaying the enzyme TMAO reductase in resting cells of the marine alteromonad NCMB 400: the enzyme was only active under anaerobic conditions with pyruvate, lactate and formate being good donors; the KmTMAO was 93 ± 16 μmol/1; TMAO reductase activity was inhibited by several N‐oxides including nitrite and nitrate, and was relatively resistant to cyanide. The relevance of conductance measurements and the significance of TMAO reduction are discussed.