Many studies have investigated the efficacy of replacing fishmeal and fish oil with black soldier flies (BSF) as a more local and sustainable resource on the growth performance of fish, although research is lacking on the effects of BSF on their gut microbiome. Furthermore, oil and chitin are commonly removed from BSF to increase the protein content and nutrient digestibility of BSF, although these extracts may improve gut health. Rainbow trout (30g) were fed for 84 and 168 days with diets based on fishmeal and a combination of fishmeal and soybean meal, respectively, to challenge the fish. Triplicate tanks of fish were fed one of seven diets that included a control, 5 and 10% full-fat BSF (FF05 and FF10), 5 and 10% defatted BSF (DF05 and DF10), 4% BSF oil (OIL) and 1% BSF chitin (CHTN). At each timepoint, the faeces and mucosa were collected, extracted, amplified and sequenced on the Illumina MiSeq platform using the V3-V4 region of the 16S rDNA. At day 84, Firmicutes were lower in the mucosa of fish fed the DF05 diet than the DF10 and OIL diets and the opposite was found for Proteobacteria (Fig. 1 ). Specifically, the genus Lactococcus was significantly less abundant in the fa eces of fish fed the DF05 diet compared to the CNTRL and OIL diets. On day 168, Firmicutes were lower in the mucosa of fish fed the DF10 diet compared to the OIL and CHTN diets. Increased abundance of Firmicutes has been used as an indicator of a healthier gut microbiome, thus BSF oil and chitin may be beneficial additives in feeds for rainbow trout and they should not be removed from full-fat BSF due to these benefits.