none. Mass occurrence of a web-spinning sawfly Cephalcia arvensis Panz. was observed in Eastern Bohemia in a vicinity of Ceska Cermna (Nachod district) since 1982 to 1988 in a spruve monoculture 60-80 years old. Affected area of about 800 ha is about 550 to 580 m a.s.l. A bionomy and phenology of individual developmental stages were studied during the last four of an outbreak. Adults emerge at this altitude since the end of April and are on the wing until beginning of June, with peak activity at about May 10. Eggs were observed from May 10 to June 5, feeding larvae since late May to mid July with greatest feeding intensity in mid June. Mature larvae drop to the ground between June 20 and July 20. Hence, judging from the data of emergence of adults and hatching of larvae, a species discussed is represented in a studied area by its spring phenological form. During an outbreak, abundance of C. arvensis reached about 97%, with only C. falleni (Dalm.) reaching to 2.7% while abundance of an otherwise common C. abietis (L.) did not exceed 0.1%. Majority of a population developed in an annual cycle, biennial development was observed only rarely (less than 1%), and triennial development was not recorded. Oviposition and feeding of larvae is concentrated in the top parts of the tree crowns. A feeding pattern of the spring phenological form of C. arvensis generally agrees with that of C. abietis (L.) however, there are differences in a type of oviposition - a species studied oviposited one to three eggs on one needle in a row but never round a needle. Eonymphs are generally found in a layer of dark debris just under a litter and never enter mineral layer of a soil. An unidentified species of Trichogramma Westw. parasiting the eggs markedly reduced a population of the host during gradation - while at a peak of an outbreak about 20% of eggs were parasitized, by the time of retrogradation a parasitization of eggs reached up to 80%. Six species of the family Ichneumonidae were established as larval parasites, i.e. Ctenopelma lucifer (Grav.), Homaspis alpina (Heinr.), Xenochesis fulvipes (Grav.), Ctenopelma funebris (Habrem.), Xenoschesis nigripes, Schm., and Homaspis narrator (Grav.), together with two species of the Tachinidae - Myxexoristops bicolor (Vill.) and Pseudopachystylum goniodes (Zett.). Phaonia errans (Meig.) and Ph. angelica (Scop.) were recorded as predators of eonymphos. This spectrum of parasites is very different from that of C. abietis. The issues of voltinism in discussed species of parasites and predators are also discussed.