In 1976-1984 the voltinism of web-spinning sawflies of the Cephalcia Pz. genus was studied at five localities of different heights above sea level (580 m - 840 m a.s. l.) situated in the Czech Socialist Republic. At the same time, different larval parasites, mainly tachina and ichneumonids, were hatching in the field breedings of web-spinning sawflies. In the course of nine years, 237 individuals of Olesicampe monticola (Hdwg) and 148 individuals of Sinophorus crassifemur, (Thom.) were obtained as specimens of parasitizing ichneumonids. It was demonstrated by the trials that at all test localities O. monticola flies produced partial subpopulations with different duration of evolution, similarly like the host. The evolution of ichneumon flies lasts one to three years, the occurrence of three-year populations is in general rare. In the given period of investigation, the two-year subpopulation had the highest population density (in nine cases out of 12 pest larval drops in different periods of summer). The one-year population ranked as the second (it dominated in one case out of twelve larval drops). The numbers of hatched imagoes of this species increase with the heights above sea level: there is already at the height zone above 700 m a.s.l. the evolutional optimum of the given species. The hatching of imagoes in the open nature took place in the period of investigation during the month of June, exceptionally until mid-July, with the maximum frequency of hatching from 15 to 25 June. The species Sinophorus crassifemur (Thoms.) also occurred as a larval parasite of weg-spinning sawflies at all localities of observation. At all heights above sea level it had mostly one-year development, two-year development ranked as the second. Three-year evolution was found to be an exception (four specimens). It can be stated according to the number of hatched imagoes that the evolutional optimum of this species occurs at the heights lower than 700 m a.s.l. Its accuracy at localities above 800 m a.s.l. was sporadic during the period of investigation. The hatching of S. crassifemur takes place mostly until June, with the highest frequency from 10 to 20 June. This polyphageous species seems to be more likely thermophilic. none