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All aphids have an obligatory, intimate association with microorganisms, either yeasts or bacteria, with the exception of the phylloxerines. This association has evolved to the extent of being required for the livelihood of both partners, neither able to live apart from the other. Such associations have been termed symbiosis; living together for mutual benefit. Here, the microorganism is referred to as the symbiont and the aphid as the host. The symbiont is obligatorily intracellular, with the exception of the secondarily acquired yeasts which are free-living in the haemolymph. They are found within cells apparently specialized to house them. These cells are termed mycetocytes; aggregates of mycetocytes are a mycetome. While mycetocytes generally contain but a single morphological form of symbiont it has been demonstrated that more than one populations of mycetocyte may comprise a mycetome. As a consequence the mycetome may contain several morphologically distinct symbionts. Within the aphids the manifestation of this multi-symbiont status is widespread; there are many examples of two symbionts and a few with, but none greater than, three. This section deals especially with the intracellular symbionts of aphids. Since research involving the free-living yeast symbionts has been meager, this group of symbionts will not be discussed further. The ultrastructure, chemistry and biochemistry of the symbionts and their interaction with th mycetocyte will be discussed. Directions for future research will be suggested.