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We recently reported that larval stage Aedes aegypti and several other species of mosquitoes grow when living bacteria are present in the gut but do not grow when living bacteria are absent. We further reported that living bacteria induce a hypoxia signal in the gut, which activates hypoxia-induced transcription factors and other processes larvae require for growth. In this study we assessed whether other types of organisms induce mosquito larvae to grow and asked if the density of non-living microbes or diet larvae are fed obviate the requirement for living organisms prior results indicated are required for growth. Using culture conditions identical to our own prior studies, we determined that inoculation density of living Escherichia coli positively affected growth rates of Ae. aegypti larvae, whereas non-living E. coli had no effect on growth across the same range of inoculation densities. A living yeast, alga, and insect cell line induced axenic Ae. aegypti first instars to grow, and stimulated similar levels of midgut hypoxia, HIF-α stabilization, and neutral lipid accumulation in the fat body as E. coli. However, the same organisms had no effect on larval growth if heat-killed. In addition, no axenic larvae molted when fed two other diets, when fed diets supplemented with heat-killed microbes or lysed and heat-killed microbes. Experiments conducted with An. gambiae yielded similar findings. Taken together, our results indicate that organisms from different prokaryotic and eukaryotic groups induce mosquito larvae to grow, whereas no conditions were identified that stimulated larvae to grow in the absence of living organisms., Author summary Studies conducted in our laboratory indicate that several species of mosquito larvae require living bacteria in their gut to grow. Previous results also show that living bacteria reduce gut oxygen levels below 5% and that bacteria-induced gut hypoxia functions as a signal that activates several processes in larvae with growth functions. Here we report that bacterial density affects larval growth rates and that other living organisms besides bacteria induce larval stage Aedes aegypti and Anopheles gambiae to grow. In contrast, the requirement for living organisms was not altered by the diet larvae were fed. These findings are of fundamental interest because they show that several types of organisms in the gut activate the same growth-related processes when viable but none do so when non-viable. Reported results also have applied implications for producing axenic adult mosquitoes for use in different types of studies.