association between organisms of two different species in which each is benefited. Mutualistic arrangements are most likely to develop between organisms with widely differing living requirements. The partnership between nitrogen-fixing bacteria and leguminous plants is an example, as is the association between cows and rumen bacteria (the bacteria live in the digestive tract and help digest the plants eaten by the cow). The associations between tree roots and certain fungi are often mutualistic (see mycorrhiza.)

Intestinal flagellated (flagellate) protozoans and termites (termite) exhibit obligative mutualism, a strict interdependency, in which the protozoans digest the wood ingested by the termites; neither partner can survive under natural conditions without the other.

Acacia ants (Pseudomyrmex ferruginea) inhabit the bull-horn acacia (Acacia cornigera), upon which they obtain food and shelter; the acacia depends on the ants for protection from browsing animals, which the ants drive away. Neither member can survive successfully without the other, also exemplifying obligative mutualism.

The yucca moth is dependent on the yucca plant and vice versa: the moth acts as pollinator at the same time that she lays her eggs in the seed pods of the yucca; the larvae hatch and feed on some but not all the seeds. Both organisms benefit: the plant is pollinated, and the moth has a source of food for its larvae.