Some Tetraponera ants (Formicidae, Pseudomyrmecinae) subsist almost entirely on amino acid deficient honeydew secretions of pseudococcids and harbour a dense aggregation of bacterial symbionts in a unique pouch–shaped organ at the junction of the midgut and the intestine. The organ is surrounded by a network of intruding tracheae and Malpighian tubules, suggesting that these bacteria are involved in the oxidative recycling of nitrogen–rich metabolic waste. We have examined the ultrastructure of these bacteria and have amplified, cloned and sequenced ribosomal RNA–encoding genes, showing that the ant pouch contains a series of close relatives of Flavobacteria and Rhizobium, Methylobacterium, Burkholderia and Pseudomonas nitrogen–fixing root–nodule bacteria. We argue that pouch bacteria have been repeatedly ‘domesticated’ by the ants as nitrogen–recycling endosymbionts. This ant–associated community of mutualists is, to our knowledge, the first finding of symbionts related to root–nodule bacteria in animals.