Abstract. – A wealth of information remains to be discovered on the functional and evolutionary significance of microbial interactions within hosts. In this review, the simple digestive tract symbioses of the tsetse fly (Diptera: Glossinidae) and the European medicinal leech (Hirudinea: Hirudidae) are described and promoted as powerful model systems for the examination of interspecific microbial relations. The comparative analyses of these two systems can highlight universal themes of microbial associations or particularities outfitted for a specific symbiotic system. This review concludes with areas of future research where the examination of concerted actions between microbial species may prove fruitful, such as in the exchange and breakdown of chemicals as well as in the regulation of microbial density. A brief description of the microanalytical tools and techniques that are available to advance our understanding of how different microorganisms interact within a host ecosystem is also included.