Beta-N-acetylglucosaminidase activity was measured fluorometrically in subsamples of a cultivation medium containing single Daphnia pulicaria. Immediately after the animals moulted, a 40-100-fold increase of activity was observed. The increase was proportional to cladoceran body length (r2 = 0.94) and was ascribed to release of their moulting enzyme. The moulting enzyme dissolved in water (in the 2.5 d. The saturation curve, estimated in adult females, followed Michaelis-Menten kinetics (V(max) = 2.68 mumol liter-l h-1; K(m) = 56.7 mumol liter-1). Release of the crustacean moulting enzyme could enhance the activity of dissolved beta-N-acetylglucosaminidase in a reservoir.