Techniques were developed to collect the periphyton on submerged wood of the River Red Gum (Eucalyptus camaldulensis DEHNH., Myrtaceae) and determine the activity of some key extracellular enzymes associated with these biofilms. Periphyton was grown on wooden tiles placed in the photic zone and the aphotic zone of the water column of two billabongs in south-eastern Australia, and enzyme activities measured with p-nitrophenyl derivatives as substrate analogues. Rates of enzyme activity were calculated per unit periphytic organic-matter (as ash-free dry weight, AFDW) and per unit tile surface area. Phosphatase activity was maximal at either pH 6-7 and 9-10 depending upon site, whereas leucine aminopeptidase and beta-D glucosidase activity were maximal at pH 7-9 and 5-6, respectively. V(max) values for alkaline phosphatase (40-84 nmol mg AFDW-1 h-1) and aminopeptidase (15-556 nmol mg AFDW-1 h-1) were greater than for beta-D glucosidase (6-9 nmol mg AFDW-1 h-1). A similar pattern was evident when rates were expressed on a per cm2 basis. Alkaline phosphatase and aminopeptidase activities were generally greater in the photic zone than the aphotic zone, but there was little effect of position on beta-D glucosidase activity. The K(m) values for alkaline phosphatase, aminopeptidase and beta-D glucosidase ranged from 220-811 mumol l-1, 158-2550 mumol l-1 and 200-826 mumol l-1, respectively. There were few consistent differences in the K(m) values of photic-zone and aphotic-zone periphyton. We conclude that the periphyton associated with submerged wood in billabongs possess active extracellular enzymes, seemingly with a poor affinity for their substrate.