The activity of the molting enzyme chitobiase in the medium surrounding individuals and populations of planktonic crustacea was investigated. Two applications of this enzyme assay are presented as methods of estimating development rates in crustacean zooplankton. The correspondence between elevated chitobiase activity in the medium and the presence of exuviae was confirmed in seven freshwater cladoceran, and one freshwater and six marine copepod species. This biochemical cue of the molting event was applied as a method of estimating the proportion of animals molting in a defined period of time (development time). An estimate of 'Daphnia magna' (2,000-2,100 [mu]m size class) development time was in close agreement with that derived by conventional incubations (70.3 versus 75.1 hours respectively). Chitobiase activity in the medium was found to vary with body length in six freshwater cladocerans and six marine copepod species. Although the slopes of species specific regressions differed, a significant common relationship was found (log10 [chitobiase activity] = -1.19 + 0.89 log 10 [body length], r2 = 0.79, p < 0.0001). Under steady state conditions in laboratory cultures, the rate of decay of chitobiase in the medium was balanced by its rate of production by molting animals. The rate of decay of the enzyme in the absence of animals was therefore also its rate of production, which is a measure of the average rate of development of the crustacean zooplankton community. Development times for a ' Daphnia magna' culture (2 determinations) and a 'Ceriodaphnia ' sp. - 'D. magna' mixed culture were 65.4 vs. 62 hours, 59 vs. 67 hours and 46.6 vs. 50 hours, as measured by this application of the chitobiase assay versus conventional molt rate determinations respectively.