Laboratory studies on cyanobacteria‐zooplankton interactions have largely focused on the inadequacy of cyanobacteria as a food source. Some features of cyanobacteria can be regarded as anti‐herbivore defences. Large colonies ofAphanizomenon, AnabaenaandMicrocystiscannot be handled by zooplankton, but do not interfere seriously with the filtering process. Small colonies and filaments, however, may cause severe inhibition of the feeding process by mechanical interference. This reduces zooplankton growth, reproduction, and survival. Copepods, rotifers, andBosminaare less affected by mechanical disturbance than cladocerans. If ingested, some cyanobacteria may be poorly digested or may not provide essential nutrients. Some cyanobacteria are reported to be toxic to zooplankton. Several strains ofMicrocystisproduce an endotoxin, but the amount of toxin produced differs among strains and with the condition ofMicrocystis. Zooplankton encountering toxic cells cease feeding. Some evidence of an extra‐cellular herbivore deterrent was found forAnabaena. Thus, cyanobacteria may either be individually protected or may create an environment that is unfavourable for efficient grazers (e.g.,Daphnia).