Many ants live in complex mutualistic or parasitic relationships with other insects or plants, some of which are classical examples of the mutual benefit of symbiosis. However, only in the past few years have new insights into the symbiosis of ants and microorganisms been reported. Examples are the symbiosis of ants of the genus Camponotus with intracellular bacteria present in their midgut, and the tripartite relationship of ants of the tribe Attini with an extracellular bacterium essential for the cultivation of their fungus gardens. The analysis of the parasitic and mutualistic interactions of these organisms will allow interesting insights into the evolution of symbiosis and possibly lead to novel strategies of pest control.