A detailed account is given of studies carried out in several localities in south-western Slovakia in 1969-70 on the parasites of Mamestra brassicae (L.), Autographa gamma (L.) and Agrotis (Scotia) segetum (Schiff.) on cruciferous crops, together with notes on their morphology, bionomics and habits.Trichogramma evanescens Westw. parasitised 10-40 and 57-58% of the eggs of M. brassicae in the two generations, respectively, and held the population in check to a considerable extent. Telenomus sp. also attacked the eggs, but to a lesser degree. Some 11 species of parasites were reared from the larvae of M. brassicae; parasitism ranged from 7.3% in 1969 to 37.3-45.1% in 1970, when the host populations were low. The most abundant of the parasites were Exetastes cinctipes (Retz.), Microplitis mediator (Hal.) and Ernestia consobrina (Mg.). The level of parasitism appeared to be density-dependent. Parasites were reared only from the larvae in the case of Autographa gamma, populations of which were small compared with those of Mamestra brassicae. Up to 56.6% of the larvae were parasitised, the most abundant of the 13 species of parasites being Copidosoma (Litomastix) truncatellum (Dalm.), Voria ruralis (Fall.) and Hyposoter didymator (Thnb.). Rogas circumscriptus Nees was relatively abundant in 1970. Some 16 species of parasites were reared from the larvae of Agrotis segetum in 1968 and 1970; one of these, Siphona collini Mesnil, had not previously been recorded from Czechoslovakia or from this host. Total parasitism reached 12.5-13%. Macrocentrus collaris ( Spin.) was the dominant parasite. Considerable numbers of larvae died from causes other than parasitism. Various diseases, including one caused by a fungus of the genus Entomophthora (Tarichium), killed up to 34.5-40 and 55.8% of the larvae in 1968 and 1970, respectively. none