Rubber additives manufacture yields wastewaters with recalcitrant and/or toxic benzothiazole compounds. Biodegradation of such compounds was investigated in fed-batch systems. 2-Mercaptobenzothiazole (MBT) was best degraded by a mixture of MBT-history and non-MBT-history sludge. Concentrations up to 200 mg.l(-1) were removed. From 100 mg.l(-1) onwards, high percentages of the recalcitrant disulphide were accumulated in the sludge. MBT slowed down the biodegradation of benzothiazole-2-sulphonate. MBT and benzothiazole did not mutually influence their degradation. Under some experimental conditions high levels of unidentified so-called polar compounds were formed.