House flies are disease vectors, carrying human pathogens which include Escherichia coli and Vibrio cholera. To explore the use of chitosan as a bioinsecticide, we evaluated the effects of a chitosan-amended diet on Musca domestica (house fly). We first conducted longevity experiments to understand the impact of chitosan on house fly longevity. We confirmed that chitosan diet amendment is associated with reduced longevity and that this is not due to starvation. We then extracted fly microbiome DNA and used 16S ribosomal RNA gene amplicon sequencing and quantitative PCR to assess the composition and load of the microbiome for flies fed chitosan-amended diets compared to controls. Diversity of the chitosan-fed fly microbiomes was lower than the control, with significant dissimilarities in community composition. Chitosan-fed flies showed lower Ralstonia relative abundance but increased relative abundance of Serratia. Both control and chitosan-fed flies had highly uneven communities, but the control flies were dominated by genera Ralstonia and Providencia, while the chitosan-fed flies were dominated by genera Serratia, Kosakonia, and Providencia. Contrary to our expected results, chitosan-fed flies also contained 56% more bacteria compared to controls. Gut microbiome changes appear to result from chitinolytic bacteria becoming more relatively abundant, and our results suggest that chitosan-amended diet alters the house fly microbiome resulting in higher fly mortality.