Biological nitrogen (N) provisioning is a seminal function of the gut microbes in several terrestrial insects, given the unbalanced carbon (C) and N ratios of their diets. Although freshwater insects face comparable dietary N limitations like terrestrial insects, little is known about this function by their gut microbiomes. In this study, we investigated microbial nitrate reduction to ammonium pathways as possible routes of biological N provisioning in two freshwater insects; filter-feeding Hydropsychidae and grazers/collectors Baetidae. After incubation in filtered (microbe-free) artificial stream water (ASW) containing dissolved ¹⁵N-labeled nitrate (treatment) or standard nitrate (control), bulk δ¹⁵N values of treatment samples (Baetidae = 100.62 ± 10.23, mean ± S.E.; Hydropsychidae = 76.82 ± 7.20) were significantly higher than controls (Baetidae = 10.14 ± 0.12 ; Hydropsychidae = 9.03 ± 0.20) in both functional feeding groups (F _{(3, 13)} = 296, P < 0.0001). The treatment δ¹⁵N values are cautiously interpreted as reflecting uptake and incorporation of microbe-derived ¹⁵N-metabolites (¹⁵NH₄ or ¹⁵N-amino acids) into host tissues following nitrate reduction to ammonium pathways in the gut lumen. Microbial nitrate reduction to ammonium activities was assessed via the quantification of dissimilatory (nrfA) and assimilatory (nasA) nitrate reduction to ammonium gene transcripts. There were no significant differences between control and treatment groups within each insect groups. Overall, this study provides a demonstration of the feasibility of applying ¹⁵N-stable isotope analysis for investigating, potential symbiotic functions of freshwater insect gut microbiomes, despite the preliminary nature of the results.