Heydenia unicaCook and Davis,Eurytomaspp., and 5 other hymenopterous parasites of the southern pine beetle,Dendroctonus frontalisZimmerman, interact in varying degrees, both inter- and intraspecifically. In the field this interaction, which we will term aggressivenes, is displayed by femaleH. unicaandEurytomaspp. during surveying and probing. Inter- and intraspecific aggressiveness are similar but differ in intensity. The activity, size, and species of both the attacking and attacked female parasite determine the degree of aggressiveness that the attacking parasites uses. This type of interference may lower the reproductive efficiency of both the attacked and attacking insect through time loss in surveying, probing, drilling, oviposition, and reduced parasitism.