A method was developed using p-nitrophenol and p-nitroaniline derivatives to measure exoenzyme activity in turbid waters, as an indicator of biodegradation. Maximum activity occurred at or above 40 C. The response to variations in pH was complex, with some systems showing no clear optimum but others showing an optimum at either pH 7.5 or pH 9.0-10. Only 25% of the enzyme system-site combinations generated linear Line-weaver-Burke plots, indicating that many exoenzyme systems were active simultaneously in the degradation of organic detritus. The maximum rate of activity occurred with aminopeptidase (up to 107 micromol/L/d), followed by alkaline phosphatase (up to 42 micromol/L/d). There was little lipase or alpha-D glucosidase activity in the any water body, except in a highly-eutrophied lagoon. Rates were consistently higher in billabongs than in rivers, and most activity was associated with particles rather than being free in the water column.