none The effect of host and parasitoid densities on searching efficiency (a') and progeny sex ratio of the parasitoid O. concolor Szepl. was studied. The parasitoid exhibited a general decreas of a' with increase of host and parasitoid densities. The mutual interference constant showed a decrease when the actual time spent on the sting unit was considered instead of the overall exposition time. There were no significant differences in progeny sex ratio in the range of host and parasitoid densities studied.