Millipedes are thought to depend on their gut microbiome for processing plant-litter-cellulose through fermentation, similar to many other arthropods. However, this hypothesis lacks sufficient evidence. To investigate this, we used inhibitors to disrupt the gut microbiota of juvenile Epibolus pulchripes (tropical, CH₄-emitting) and Glomeris connexa (temperate, non-CH₄-emitting) and isotopic labelling. Feeding the millipedes sterile or antibiotics-treated litter reduced faecal production and microbial load without major impacts on survival or weight. Bacterial diversity remained similar, with Bacteroidota dominant in E. pulchripes and Pseudomonadota in G. connexa. Sodium-2-bromoethanesulfonate treatment halted CH₄ emissions in E. pulchripes, but it resumed after returning to normal feeding. Employing ¹³C-labeled leaf litter and RNA-SIP revealed a slow and gradual prokaryote labelling, indicating a significant density shift only by day 21. Surprisingly, labelling of the fungal biomass was somewhat quicker. Our findings suggest that fermentation by the gut microbiota is likely not essential for the millipede’s nutrition.