Toosendanin has been suggested as a potential agent for controlling zooplankton contamination in microalgal cultivation. Growth inhibitory and antifeedant influences of toosendanin on the rotifer Brachionus plicatilis were investigated. Sublethal concentrations of toosendanin (1.76-2.59 mg m(-3)) reduced population abundance of rotifers, decreased the lorica length and width, and shrank the body size of survivors notably. No changes were observed in the amictic egg sizes, although the egg production per female rotifer was decreased by toosendanin. Exposure to toosendanin for 4 h significantly decreased the rotifer chlorophyll consumption per individual, indicating the antifeedant effect on rotifers. Analysis on the digestive enzymes showed that exposure to toosendanin had no effect on cellulose activity, but reduced the activities of diastase, pepsase and tryptase in rotifer homogenate. In addition, the half saturation constant (Km) of tryptase was increased and the maximum velocity (V-max) of pepsase was decreased by toosendanin, suggesting the direct inhibition of these two proteases by toosendanin in B. plicatilis. This may contribute to the feeding deterrent property of toosendanin on rotifers. The present study heralds the very beginning of studies regarding the toxic mechanism of toosendanin on rotifers, and will accelerate its potential uses for zooplankton extermination in microalgal cultivation. (C) 2017 Elsevier Ltd. All rights reserved.