Even in the most thorough biomanipulation studies, which covertheresponse of all trophic levels to manipulation at the top ofthefood chain, the mechanisms underlying the success or failureof anexperiment often remain obscure. The case of a small eutrophicLakeWirbel in Poland is an example of how useful it could be toapplydemographic and behavioural analysis of dominant planktonicherbivores to explain the sequence of events followingbiomanipulation measures.At a certain stage of a seven-year (1988–1994)biomanipulationexperiment, rotenone was applied to remove all fish. Followingfishremoval, summer algal biomass in the edible fraction ofphytoplankton (<30 µm)="" was="" on="" average="" 2.5="" times="" lower.besides="" the="" significant="" increase="" in="" the="" density="" of="" herbivorouszooplankton="" (above="" all="" the="" largest="" cladoceran="" daphniacucullata),="" we="" identified="" three="" other="" phenomena="" underlyingtheobserved="" reduction="" in="" algal="" biomass="" which="" may="" contribute="" totheoverall="" zooplankton="" grazing="" efficiency:="" (1)="" the="" significantincrease="" in="" the="" mean="" body="" size="" of="" the="" dominant="" cladoceran="" daphnia="" cucullata,="" calculated="" for="" the="" whole="" water="" column,(2)="" theclear="" shift="" in="" the="" vertical="" distribution="" of="" daphniatowardsthe="" surface="" and="" (3)="" the="" increase="" in="" mean="" daphnia="" size="" in="" thesurface="" waters.we="" postulate="" that="" this="" type="" of="" analysis="" should="" be="" performed="" alongwith="" themeasures="" conventionally="" used="" in="" biomanipulation="" studies,first,="" asa="" tool="" to="" fully="" understand="" the="" sequence="" of="" events="" generated="" bymanipulations="" at="" the="" top="" of="" the="" food="" chain="" and,="" second,="" as="" aperfect="" opportunity="" to="" study="" population="" and="" individualresponses(e.g.="" morphological,="" life-history="" and="" behavioural)="" to="" varyingpredatory="">