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Intraspecies competition in a field population of Gregopimpla himalayensis (Hym.: Ichneumonidae) parasitic on the prepupae of Malacosoma neustria testacea (Lep.: Lasiocampidae) was investigated. The parasite oviposits the sufficient number of progeny (5 individuals/0.1 g dry weight of host) to exhaust a single host in a single attack. However, at the intensity less than 22-26 individuals/0.1 g d.w. of host, all individuals can emerge, i.e. density-dependent mortality does not occur. Within this range of intensity, survival of parasite larvae is guaranteed by diminution in body size and decreasing sex ratio. In contrast, total biomass of parasites showed a peak at 5 individuals/0.1 g d.w. of host at which a single host is exhausted. Above the intensity of 22-26, extraordinary minute individuals appeared and they died before maturation. If intraspecies competition play a role in regulation of G. himalayensis population in the field, the process is usually not through density-dependent mortality but through decreasing reproductive rate caused by decrease in the sex ratio, adult longevity and fecundity. [ABSTRACT FROM AUTHOR]