Background and Objective: Lushoto district has been an active focus of plague disease since 1980 and many pesticides were used to control rodents and fleas from 1980 to 2003 when outbreaks occurred yearly. For over seven years ago commercial Carbaryl 5% powder has been used for controlling fleas in the area. However, there is no current research to substantiate its effectiveness either in the laboratory or in the field. Methods: Immature stages of Xenopsylla brasiliensis were collected from two villages in Lushoto and transported to Sokoine University of Agriculture (SUA), Morogoro and reared in an insectary to stock colony. Known weights of commercial Carbaryl 5% powder were thoroughly mixed with known weights of clean fine sand as to obtain a final concentration of 0.05% (WW) of Carbaryl/sand mixture. The same concentration was suspended in 50cc distilled water and pieces of filter paper were soaked in the solution, left at room temperature until the suspension was fully adsorbed and the papers were left to dry. Adult fleas of mixed ages and of both sexes were obtained from the stock colony and exposed to both Carbaryl/sand mixture and Carbaryl-adsorbed filter papers for various periods of time. Field trials were carried out at Manolo and Viti villages where house and rodent flea indices were determined before and after dusting with commercial Carbaryl 5% powder. Results: In the laboratory tests, 100% mortality occurred at 90 minutes exposure in Carbaryl/sand mixture experiments and at 35 minutes exposure to Carbaryl-adsorbed filter papers. LT50 in both sets of exposure was 48.2 min and 23.1 min in Carbaryl/sand mixture and Carbaryl-coated filter papers tests respectively. In field trials, X. brasiliensis and Pulex irritans were the most abundant flea species. Post-dusting flea populations were significantly lower in treated than in control houses (p=0.028). House flea indices dropped from 7.7 to 0.33 and 37.8 to 0 in Viti and Manolo villages respectively at 3 months post-dusting. Conclusion: Commercial Carbaryl 5% powder in current use was still effective against potential flea vectors in Lushoto.