The gut microbiota of animals are the most common and complex symbionts. They affect host metabolism, diseases, and immunity. Microorganisms enter the gut with food, we are interested in their communities and assembly mechanisms. Compared to omnivore, nectivorous butterflies have simple gut microbiota for studying gut microbiota assembly. Community assembly process including dispersal limitation, environmental filtering, and competitive exclusion. Diverse yeast populations in nectar enter the gut without dispersal limitation. However, the microbiota in nectar and gut are different, which is attributed to strong environmental filtering. We know few about competitive interactions among yeast populations in guts. Exploitative competition (rapid resource utilization) and interference competition (secreting inhibition substance) are two major competition strategies. In this study, the fungal communities in Euploea butterflies’ guts and nectars were compared by high-throughput sequencing. We found Candida corydali (Cc) was the dominant yeast in midguts but rare in nectars and dominant nectar yeasts did not colonize guts. These demonstrated the phenomenon of environmental filtering. Fifty-five Cc strains were determined as killer yeast.Candida corydali had weaker exploitative competition ability than sensitive yeasts. The co-culture experiment revealed killer yeast suppressed exploitative competition ability of sensitive yeast by interference competition. This study explores the competitive interaction of yeasts in butterflies’ guts. Nectar yeasts enter the gut without dispersal limitation. The environment of guts excludes unfitted species. The remaining species suppress other competitors by interference competition, becoming dominant species in yeast community of Euploea butterflies’ guts.