The performance of mesophilic (35 degrees C; referred to as R1) and thermophilic (55 degrees C; R3) laboratory activated sludge processes (ASPs) as well as ASP with a fluctuating temperature (27-56 degrees C; R2) was compared. During the 124-day runs, in R1 and R3 hydraulic retention time was gradually reduced from 18 h to 3 h, corresponding to an increase in volumetric loading rate from 2 to 10 kg soluble COD m(-3) d(-1); in R2 hydraulic retention time was gradually reduced from 18 to 4.5 h, corresponding to an increase in volumetric loading rate from 2 to 7.5 kg soluble COD m(-3) d(-1). R1 removed on average 85% of soluble COD (GF50-filtered) that was approximately 10% more than R3. In R2 SCOD removal was dependent on the operating temperature, being comparable to R1 and R3 at respective temperature. However, the COD for 0.45 microm-filtered (bacteria-free) effluent samples was lower for R3 than for R1, indicating the role of free bacteria on effluent quality. Furthermore, 24 h post-aeration of R3 effluent at 35 degrees C decreased SCOD (GF50-filtrated) markedly (43% removal), whereas at 55 degrees C no SCOD removal occurred, which suggest mesophilic post-treatment ability to remove thermophilically recalcitrant matter or, more probably, the ability of free bacteria to aggregate more efficiently under lower temperatures. The results indicate that temperature may not be as crucial a factor in high temperature biological wastewater treatment as previously believed. On the other hand, in thermophilic ASP the importance of solids separation is emphasized.