The cowpea beetle, Callosobruchus maculatus (Fabricius, 1775) (Coleoptera: Chrysomelidae) is a major pest of stored legumes seeds such as cowpea, chickpea, lentil, soybean, bean in both Turkey and many other countries. In this study, the fumigant effect of the essential oil obtained from rosemary [Rosmarinus officinalis L. (Lamiales: Lamiaceae)] plant was investigated against eggs, first and last-instar larvae and pupae of C. maculatus in chickpea. The study was conducted at Selcuk University, Faculty of Agriculture, Department of Plant Protection Entomology Laboratory under conditions at 28±2°C temperature, 55±5% relative humidity and fully dark conditions in 2013. As parallel to the increase of exposure time and applied dose, an increase occurred in mortality of immature stages of C. maculatus. First instar larvae and eggs were the most susceptible stages to rosemary oil while the pupae and last-instar larvae were the most tolerance stages. LC50 values for eggs, first and last instar larvae and pupae were found as 34.57, 27.64, 60.39 and 60.34 μl/l for 24 h exposure time of rosemary essential oil vapor, respectively. At 50 μl/l air dose of rosemary oil and exposure time of 24 h, mortality rates of eggs, first instar larvae, last instar larvae and pupae were determined 100, 91.33, 56 and 46%, respectively. In conclusion, rosemary essential oil showed high fumigant toxicity against the immature stages of C. maculatus.