The digestive tract of most animals is inhabited by a complex community of microbes, including bacteria, yeasts, protozoans, and viruses. This gut microbiota influences host physiology from metabolism and immunity to behavior. Bacteria-focused research is more abundant in the published literature when compared to other microbes. Drosophila melanogaster, with its relatively simple gut microbiota comprised mainly of bacteria and yeasts, is an excellent model for studying the lesser explored role of yeasts on host physiology. Yeasts are often provided to flies as nutrients, but their role in the gut microbiota is poorly understood. I studied the effects of Saccharomyces cerevisiae (the yeast commonly used in lab settings) and Lachancea kluyveri (a yeast originally isolated from the gut of Drosophila spp.) on fly development. I reared D. melanogaster as axenic (free of microbes) and gnotobiotic (with a known yeast species in their gut) flies and recorded their pupation and eclosion times. Both yeasts on their own reduced larval development time by 20 % when compared to axenic (free of microbes) flies, recreating the effect of having an intact microbiota. The effect is not purely dietary, as heat-killed yeasts and nutritional supplements (amino acids and B-vitamins) reduce development time only by c. 10 %. Future studies will focus on identifying the mechanisms underlying the effects of gut yeasts on D. melanogaster physiology.