Thermophilic aerobic digestion (TAD) of a model agricultural waste slurry was carried out in a continuously stirred tank reactor for a total of 156 h at 55 degreesC. The pH of the digestion was either unregulated or regulated at 6.0, 7.0, 8.0, 9.0 and 9.5 at 0.5 volume of air per volume of waste slurry per minute (vvm) aeration rate. The effect of aeration rate was studied at 0.1, 0.25, 0.5 and 1.0 vvm at pH 7.0. Development of thermophilic populations and amylase and cellulase enzyme activities in the digesting slurry were studied. Thermophiles developed rapidly to reach peak populations in 24 h or earlier. After 96 h, thermophiles were present mostly as spores. They were only slightly affected by aeration rate, but more by digestion pH. Amylase developed rapidly to reach peak activity before declining as rapidly. activity Cellulase was considerably affected by aeration rate but not digestion pH. Amylase profile closely reflected development of microbial activity, and so the progress of TAD. It may be useful as a control tool, either alone or in combination with other process data, for monitoring the TAD of complex waste slurries. (C) 2003 Published by Elsevier Ltd.