The body size of an animal plays a crucial role in determining its trophic level and position within the food web, as well as its interactions with other species. In the symbiosis betweenTermitomycesand fungus-growing termites, termites rely on nutrition of fungal nodules produced byTermitomyces. To understand whether the size of termites and fungal nodules are related to their partner specificity, we quantified the size of termite farmer caste, and the size and density of nodules in termite nests of four genera of fungus-growing termites, and identified their cultivatedTermitomycesfungus species based on internal transcribed spacer regions and partial large subunit ribosomal RNA gene sequences. The results showed that the size and density of fungal nodules were different amongTermitomycesclades and revealed a constant trade-off between size and density among clades. The nodule size of each clade has low variation and fits normal distribution, indicating that size is a stabilized trait. Moreover, we found larger termite genera cultivatedTermitomyceswith larger but less numerous nodules. Based on these results, we concluded that there is a size specificity betweenTermitomycesand fungus-growing termites, which may lead to diversification ofTermitomycesas adaptations to different termite genera.