Despite its importance for biowaste management and feed and fertilizer production, fundamental insights in the digestion processes of black soldier fly larvae (BSFL) are lacking. To gather additional data on the transition of different size particles, this study assessed the ingestion and egestion rates of various sized microplastics in the BSFL digestive tract. BSFL were exposed to a plant-based rearing diet (75% moisture content, 14% protein, 3% fat, 60% carbohydrates) spiked with a soluble dye (0.05% Blue1), two sizes of fluorescent microplastics (15 and 58 µm, ≈ 50,000 particles/g diet) and one size of palladium-doped nanoplastics (200 nm). During ingestion on the spiked diet (10-300 min) and egestion on the clean diet (60-1,080 min), larvae were regularly removed from the diet, dissected and the foremost dye and microplastics position (i.e. anterior, mid, posterior and hindgut) were determined. The dye position was determined with a stereomicroscope and the foremost microplastic position and total microplastic number with a fluorescent microscope. Nanoplastics were quantified in whole gut samples via ICP-MS detection of palladium-doped particles. All particles were ingested by BSFL, but discernible variations were observed based on particle size. The soluble dye passed through the digestive tract the fastest, followed by the 15 µm and the 58 µm microplastics, respectively. Ingested microplastic numbers were a magnitude higher for 15 µm (≈ 100/larvae) than 58 µm (≈ 3/larvae) microplastics. Relevant for product safety as animal feed, after 1080 min of egestion, larvae contained low amounts of 15 µm microplastics (4/larvae), while no 58 µm microplastics were present and nanoplastics were below the detection limit. Thereby this work contributes to fostering a deeper understanding of BSFL digestion with potential implications for improving bioconversion.