Fungus-growing termites are ecologically important animals in tropical Africa and Asia. Especially in dry savannas, they contribute to local carbon and mineral recycling and alter soil physical properties, thus facilitating the success of many plant species. This, in turn, has indirect impacts also on animals that may e.g. benefit from improved food supply and quality. The success and ecological significance of fungus-growing termites arise from their exosymbiotic relationship with the fungal genus Termitomyces. Termites cultivate fungal symbionts within specialized compost structures in their underground nests where the mycelium assists in degradation of plant matter collected by the termites, thus providing a constant food supply for the large termite colonies. Symbiotic food processing is especially advanced in the termite genus Macrotermes which construct large above-ground soil structures – termite mounds – to enhance ventilation of the below-ground nests and to provide a favorable microclimate for fungal growth even in arid savanna environments. The aim of this thesis was to study interactions between Macrotermes termites and their Termitomyces symbionts in the semiarid Tsavo Ecosystem in Southern Kenya. We assessed the local diversity of the host insects and their fungal symbionts and produced an up-to-date phylogeny of the fungal symbionts based both on our new results and previously published DNA data. We found that the Macrotermes–Termitomyces diversity in the Tsavo Ecosystem involves two host species and three symbiont species that occur in different combinations, and the frequencies of different associations vary over the landscape. Studies on mound architecture and symbiont diversity revealed correlations between the size and type of above-ground mounds and specific host-symbiont combinations. These were linked to architecturally induced differences in nest temperatures, suggesting that different Termitomyces species may differ in their ranges of tolerable growth temperatures. Stable isotope studies provided important new information on the nutritional role of Termitomyces for Macrotermes colonies. Termitomyces promotes the nutrition of the host insects directly, as highly nitrogenous food for queen and young larvae, and indirectly, by decomposing plant matter that is eaten by workers, soldiers, and developing alates. Thereby, the fungal symbiont does not have a single universal role in the nutrition of a termite colony, but instead, different termite castes depend on the symbiosis in different ways. The isotopic imbalance of nitrogen also implied that, although the nutrition of fungus-growing termites is facilitated by the fungal symbionts, also bacterial nitrogen fixing may provide an essential complementary nitrogen source for termite colonies.