The thermophilic bacterium, Thermus species ATCC 27978, which is capable of aerobically degrading benzene, toluene, ethylbenzene, and the xylenes (BTEX), was cultured in 5-1 fermenters on a Castenholz salts-tryptone medium. This bacterium can be cultivated more conveniently at 45 degrees C, a temperature substantially lower than its optimal growth temperature (approx. 60 degrees C). Yet, the washed harvested cells from such cultures display the same initial BTEX-degrading activity as those when Thermus sp. is grown at its higher optimal temperature. Two bioreactor cultivation modes, batch and fed batch, were investigated. More biomass and more BTEX-degrading activity (assayed at 60 degrees C) were generated in fed-batch cultures than in the growth-limited batch cultures. The former yielded a biomass concentration of 2.5 g dry cell weight (DCW) 1(-1) and whole-cell degrading specific activities of 7.6 +/- 1.3, 10.1 +/- 1.9, 9.8 +/- 2.1 2.3 +/- 0.5, and 4.6 +/- 0.9 nmol degraded (mg DCW)(-1) min(-1) for benzene, toluene, ethylbenzene, m-xylene, and the o- plus p-xylenes (unresolved mixture), respectively. Although the formation of cellular BTEX-degrading activity is growth-associated, a slow to moderate specific growth rate of 0.02 - 0.07 h(-1) favors the production of BTEX-degrading activity, while a high growth rate, of the order of 0.16 h(-1), is detrimental to its production. The washed harvested Thermus sp. cells were capable of degrading BTEX over a broad range of thermophilic incubation temperatures, 45-77 degrees C.