Abstract.1. The relationship between cyanogenesis in bracken fern and the insect fauna feeding on the plant was investigated over a 3 year period. The most common insects between May and July, while cyanide levels were high, were the sawfliesStrongylogaster impressataProvancher,S.multicinctaNorton,Aneug‐menus fzavipes(Norton), the aphidMacrosiphum euphorbiae(Thomas) and a microlepidopteran species ofMonochroa.2. Collections of insects from cyanogenic and acyanogenic fronds showed significantly fewer sawflies on the cyanogenic fronds. The aphid and the microlepidopteran were randomly distributed with respect to cyanogenicity.3. Feeding tests for two of the sawfly species (SimpressataandSmulticincta) showed that larvae grew more slowly and had a higher mortality when raised on cyanogenic fronds than on acyanogenic ones.4. Field collected cyanogenic bracken fronds were found to have sustained less damage from chewing herbivores compared with acyanogenic frond