Benzothiazole (BT), 2-mercaptobenzothiazole (MBT), and 2-(methylthio)benzothiazole (MTBT) were determined as degradation products of the fungicide 2-(thiocyanomethylthio)benzothiazole (TCMTB) in tannery wastewater and are shown to be incompletely removed (75%) in an anaerobic and aerobic wastewater treatment pilot plant. Average total concentration of these benzothiazoles is 5.7 μmol L -1 in the untreated wastewater and 1.4 μmol L-' after aerobic treatment. Aerobic batch degradation tests revealed that TCMTB is transformed to MBT. MBT is primarily methylated to MTBT, which was not further degradable. BT was aerobically degraded along unknown pathways. Potential effects of benzothiazoles entering the aquatic environment are illustrated by luminescence inhibition of Vibrio fischeri (= Photobacterium phosphoreum ; EC 50 between 0.03 μmol L -1 for TCMTB and 32 μmol L -1 for BT) and growth inhibition of the same organism with TCMTB and MBT. MBT, BT, and MTBT at concentrations of 0.1- 0.2 μmol L -1 inhibit nitrification on sediment columns and mixed culture respiration determined as BOD (0.6-11 μmol L -1 ). It is concluded that 2-substituted benzothiazoles employed in industrial processes are not completely removable by biological wastewater treatment and are of concern for aquatic environment due to their limited biodegradability and potential toxicity.