Abstract The biodegradability of tetrachlorobenzyltoluenes (TCBTs) and polychlorinated biphenyls (PCBs) was investigated in activated sludge and in batch cultures of the PCB-degrading microorganism Alcaligenes sp. JB1. There was no evidence for significant biodegradation of either TCBTs or PCBs in activated sludge. In contrast, cultures of strain JB 1 did degrade PCB 40, but again there was no evidence for degradation of TCBTs. Although no final conclusions can be drawn from these results about the biodegradability of TCBTs, they do indicate that TCBTs are environmentally persistent compounds.