none Various aspects of the biology of Coeloides sordidator, an ectoparasitoid of pine weevils of the genus Pissodes, were studied. When reared in the laboratory on P. castaneus, females preferred to oviposit on third and fourth instar larvae in galleries, but ovipositions were also observed on second instars and prepupae. Pupae were not accepted as hosts. At 23 degree C and a 16 h day length, pre-imaginal development lasted 15-20 days. For each parasitoid population tested, between 8 and 28% of the individuals diapaused as prepupae in cocoons. The rate of diapause was not influenced by the photoperiod during larval development. In contrast, diapause was induced by a short day photoperiod on parents before and during oviposition and by low temperatures during the larval stage. Diapause termination was achieved by cold storage at 2 degree C for at least 8 weeks. Rearing of C. sordidator on P. castaneus under natural conditions showed that in the Swiss Jura it is mainly univoltine, with less than 10% of the parasitoids emerging for a second generation, When C. sordidator females were offered healthy P. castaneus larvae and larvae parasitized by the braconid endoparasitoid Eubazus semirugosus, there was no evidence that they were able to discriminate between healthy and parasitized hosts. The potential of C. sordidator as a biological control agent against the North American species P. strobi is discussed.