[Objectives]  The parasite Microplitis mediator is an important biological control for the cotton bollworm Helicoverpa armigera. To analyze the impact of parasitism by M. mediator on the behavior of H. armigera larvae we compared the behavior of parasitized and unparasitized larvae in a green house. [Methods]  Using the fixed-interval time scan sampling method, the feeding behavior and spatial displacement of unparasitized and parasitized H. armigera larvae were investigated and recorded from 1 to 10 days post-infection by M. mediator. [Results]  Parasitized larvae spent significantly  (P < 0.01) less time feeding on cotton plants than unparasitized larvae, and, spatial displacement was significantly (P < 0.01) higher in parasitized larvae than in unparasitized larvae 1-2 days after infection. Spatial displacement among parasitized larvae declined significantly (P < 0.01) over time with significantly less parasitized larvae found on cotton plant bolls than on cotton flowers (P < 0.05). Later in the developmental period significantly more parasitized larvae were found on the abaxial blade of cotton leaves than unparasitized larvae (P < 0.01). [Conclusion]  Parasitism by M. mediator had a significant impact on the feeding behavior and within-plant distribution of H. armigera larvae.