Application of chemical insecticides poses a wide range of problems in the environment. They are non specific and are implicated as carcinogens hence the need to search for an eco-friendly biological control agent such as the use of Bacillus thuringiensis habouring insecticidal crystals. In this study, Bacillus thuringiensis were isolated from different soil types in Zaria. The 12 isolates from these obtained from different soil types were screened for the presence of cry gene by PCR using primers specific for cry2 and cry4 (diptera-active cry genes). Of the 12 isolates, 1 isolate had only cry2 gene, 4 isolates had cry4 gene, 1 isolate had both of the genes while 6 isolates had none of the two genes. Bioassay to assess the insecticidal activity of the isolates was carried out using Culex quinquefasciatus and Aedes aegypti larvae using 3 different concentrations of spore crystal mixture (100, 75 and 50 ppm) alongside one control. In each case, 10 larvae of Cx. quinquefasciatus and A. aegypti were exposed to spore crystal mixture in triplicates for both. The isolates differ greatly in their larvicidal activity against the larvae of Cx. quinquefasciatus and A. aegyti. The mortality of Cx. quinquefasciatus larvae when exposed to 100ppm concentration of the isolatesâ€™ spore crystal mixture ranged between 33.33% and 96.00% while the range of mortality for A. aegyti was between 40.00% and 100.00%. The concentrations of the spore crystal mixture which kill 50% of the exposed populations in standard bioassays (LC50) were determined by probit analyses. The LC50 using Culex quinquefasciatus was between 135.95ppm and 37.48ppm while it was between 118.03ppm and 55.79ppm for Aedes aegypti. The results of this research shows that the isolates habouring the diptera-active cry gene from the soils in Zaria can serve as biocontrol agent for the control of mosquito by targeting their larvae stage hence controlling the diseases they spread.