In the year 2007, the Ministry of Health (MoH) initiated a larviciding program using Bacillus thuringiensis israelensis (Bti) to mitigate the effects of black fly bites. This study was aimed at assessing the impact of Bti on adult and larvae black fly populations. Baseline data was collected prior to Bti application and after application Larva monitoring was done using four different substrates: nylon strips, rocks, and debri. Adult monitoring was carried out by human landing catches. Data analysis included descriptive summaries, t-tests, regression and Analysis of Variance (ANOVA). The analysis also included the assessment of the effect of Bti on adult flies and Larva density on substrates. All the statistical analysis were done at 5% significance level. The results showed statistically significant differences (p <0.001) in populations of black fly before and after Bti application. Larva density was higher before Bti application and adult numbers were also high in that period. After Bti application a decrease in larva density was recorded and this associated with a gradual decrease in adult numbers. Bti had an impact on the larval population in that a decrease in larva population due to larviciding resulted in the decrease of adult population.