APYZYM(TM) assays were used to detect the presence of enzyme activity from 19 different enzymes, including three phosphatases, three esterases, three amino-peptidases, and eight glycosyl-hydrolases during co-composting of poultry manure and yard trimmings (poultry + yard trimmings). Results of this study have shown that the poultry + yard trimmings compost went through physico-chemical and biochemical changes during composting. These changes included self-heating of the compost mass, relative increases in enzyme activities, decreases in water-soluble components (i.e. water-extractable C, inorganic N, and heavy metal contents), and elimination of phytotoxicity. An overall increase in enzyme activity was observed over the course of the present experiment. Alkaline and acid phosphatase, and leucine amino-peptidase activities were high, while lipase, esterase, and esterase activities were moderate at the beginning of composting. Cystine amino peptidase, chymotrypsin, and trypsin showed no evidence of activity during the entire period of composting. Of the eight glycosyl-hydrolases, only alpha -galactosidase, beta -glucosidase, and N-acetyl-beta glucosaminidase showed any significant activity, fluctuating between low and moderate activity. The activity of sc-mannosidase was low while beta -galactosidase. beta -glucuronidase, and alpha -fucosidase remained undetected during the entire testing period). Although this enzyme test is rather preliminary, the results of this study seem to show potential usefulness of enzyme activity measurements as indices of the course of the actual composting.