none Rhynchaenus fagi; HYMENOPTERAN-PARASITES; Eubazus minutus; Parasite prevalence & ecology; BULGARIA-; Kosti State Forest; Hymenopteran parasite ecology Eubazus minutus; LIFE-CYCLE; Phenology & flight dynamics; COLEOPTERAN-HOSTS; Rhynchaenus fagi; Parasite prevalence & ecology; POPULATION-SEX-RATIO; BULGARIA-; Kosti State Forest; Parasite population dynamics & phenology, coleopteran host Beech leaves with larvae and pupae of Rhynchaenus fagi L. from 80-year-old pure beech stand on the territory of the state forest enterprise Kosti have been collected. The sexual index and the flight dynamics of the parasitoid Eubazus minutus R. have been determined. It was established that Eubazus minutus R. is the most widely spread parasitoid on the larvae and pupae of Rhynchaenus fagi L. The sexual index of Eubazus minutus R. is 0.29 with a predominance of the male specimen. In spite of this its population is exclusively vital, showing dynamic stability in the last years and plays significant role for regulating the number of Rhynchaenus fagi L. The flight of the male specimen of Eubazus minutus R. in the spring of 1992 begins on 19 May and ends on 5 June with a climax during the period 23-28 May. The flight of the female ones begins a little later, on 22 May and ends on 1 June with a climax on 24-28 May. These terms must be taken in view when eventual chemical treatment against Rhynchaenus fagi L. is carried out.