The aim of this study was to investigate the foraging behavior of Aphidius matricariae (Haliday) (Hymenoptera: Braconidae) as a biological control agent of Myzus persicae nicotianae Blackman (Hemiptera: Aphididae), a key and cosmopolitan pest of tobacco fields. To achieve a strategy for the control of this pest and a mass-rearing program of the parasitoid, host stage preference, switching, functional response, and mutual interference of A. matricariae were investigated at 25 ± 1°C, 70 ± 5% RH and 16:8 h L:D photoperiod. The parasitoid showed a preference for third- and fourth-instar nymphs of tobacco aphid in both choice and no-choice experiments. Using the Murdach's model, switching behavior was observed in A. matricariae between different density proportions of third- and fourth-instar nymphs. Further, the parasitoid exhibited a type II functional response when it was offered to third-instar nymphs of M. persicae nicotianae at six densities (2, 4, 8, 16, 32, and 64). Based on the linear regression analysis, there was a significance difference between the logarithm of per capita searching efficiency and the logarithm of parasitoid density. As the wasp density increased, per capita searching efficiency decreased. The result of this study revealed that A. matricariae is an effective agent in the integrated management of M. persicae nicotianae. In addition, application of these results can be important in mass-rearing program of A. matricariae.