The resources acquired by mosquitoes in their larval stages are critical determinants of their fitness and vector competence. Previous work has shown that both nutrients and the gut microbiome serve as resources that larval mosquitoes require to develop, and that restricted access to either resource can result in fitness penalties such as increased mortality or longer development time. However, we still lack a detailed understanding of the overall nutritional requirements of mosquitoes as well as how nutrient-gut microbe interactions affect fitness and vector competence. To begin to address these unknowns, we reared axenic larval Aedesaegypti mosquitoes on a defined diet where the abundance of major macronutrients was systematically altered. We then generated gnotobiotic mosquitoes using a simplified microbial community and reared them on a subset of diets used to rear axenic larvae. For both axenic and gnotobiotic mosquitoes, we measured several variables related to fitness including larval survival to eclosion and development time to pupation. Diets with reduced or absent protein induced severe fitness costs in axenic larvae. The presence of a single microbe marginally improved larval survival, but multi-member communities promoted mosquito development in low-nutrient conditions.