In the aerobic bacterium Comamonas testosteroni T2, the growth behaviour as well as the regulation of the toluenesulphonate (pTS) metabolism and the adaptation process arising from feed switching have been investigated in batch and chemostat culture during cultivation with acetate, pTS and mixtures thereof. In batch culture, µmax with pTS was 0.18 h-1, with acetate 0.22 h-1 and with a 1:1 mixture of pTS and acetate 0.29 h-1. Both acetate and pTS were used simultaneously in batch as well as in continuous culture thus indicating that acetate does not repress the utilisation of pTS. During cultivation with different mixtures of pTS and acetate at a dilution rate D = 0.05 h-1, the pTS metabolism became induced when pTS contributed to only 1 % of the total carbon in the substrate mixture supplied. When the fraction of pTS contributed to 5 % of the total carbon in the medium a maximum induction level was already reached. The regulation of pTS degradation was analysed in chemostat culture at a dilution rate D = 0.05 h-1 under transient growth conditions when either the feed was switched from a medium containing only acetate as carbon to one containing a mixture of acetate and pTS or when the bacterial culture growing on 100 % acetate was pulsed with 100 mg/I pTS carbon. The results showed that the adaptation period was dependent on the pTS concentration: the higher the pTS concentration in the cultivation liquid was the shorter was the lag phase until induction occured. Furthermore, the observations from the pulse experiments indicate that pTS degrading enzymes are newly produced during the adaptation period since no induction occured when an inhibitor of protein synthesis was added to the chemostat before it was pulsed with pTS.