none. A mass outbreak of the sawfly Cephalcia falleni (Dalm.) was observed in Eastern Bohemia in the eighties. The mass outbreak occurred on the mountain range of the Orlicke hory (950-1080 m above sea level) in forest stands of the "mountain spruce stand with an European mountain ash admixture" type. This area is exposed to heavy air pollution. Observations concerning the bionomics and phenology of particular pest stages and of its larval parasite spectrum were performed at this locality in the years 1986-1989, that means at the end of the mass outbreak. The mass outbreak of this sawfly resulted in the destruction of almost all needles in the spruce tree-crowns and in the mass die-back of spruce trees. Pest adults appeared at the mentioned height above sea level as soon as the snow cover disappeared in places, in the period from about the 10th April to about the 15th May, differently in particular years. In comparison with the spruce web-spinning sawfly adults, the eclosion and flitting of Cephalcia falleni adults was observed a month earlier. The egg stage lasts from the 20th April to the 5th June at the latest, the feedings of larvae take place at the given height above sea level from mid-May to early August. The overall development of his pest in the tree-crowns (including the oviposition period)lasts 6 to 10 weeks. The shedding of full-grown larvae can be observed actually throughout the whole month of July to mid-August. In 1986 (the last year when the mass of outbreak culminated) there were found up to 200 pest larvae per 1 m-2 land. The sawfly Cephalcia falleni had mostly a one-year development (9.2%) and part of the population had a two-year development (1.2%) in the period of observation. The generation with a three-year development was not observed. No egg parasite was found to infest the sawfly ova at this height above sea level. This is the case of marked incoincidence between the very early occurrence of the host ova and the later appearance of the egg parasite adults. The pest ova are laid exclusively by one ovum to older needles. The larva feeding on the branches is hardly observable because the frass is shed freely to the ground and the frass bags are not created. Only three species of larval parasites were determined. The tachina fly of the species Myxexoristops bicolor (Vill.) is the most efficient (5% parasitism), while only one specimen of the Ichneumon flies Notopygus nigricornis Kr. and Sinophorus crassifemur (Thoms.), respectively, was reared. The beginning of the tachina fly eclosion is early July, that means it overlaps with the period of the host larva shedding. The parasite development lasts mostly a year. Both species of the Ichneumon flies were also univoltine. The keys for the differentiation of the eonymphs in most species of the Cephalcia Pz. genus are described in the text of this paper.