Behavioral differences between the eastern and western strains of the alfalfa weevil,Hypera postica(Gyllenhal), in response to parasitization byTetrastichus incertus(Ratzeburg) were observed and described. Behavior was found to influence the length of time parasites remained on larvae of the host and success of parasitization. Larvae of the eastern strain generally were found to be docile in respect to parasitization. The majority of eastern larvae were parasitized successfully and judged to be more favorable hosts forT. incertus. Larvae of the western strain generally responded to parasitization with a prompt, vigorous evasive behavior and were not parasitized. This reaction to parasitism was determined to be a contributing factor inhibiting the establishment ofT. incertusin Colorado.