Eight species of Myzolecanium Beccari (Hemiptera: Homoptera: Coccoidea: Coccidae) are reported from ant nests in stem cavities of living lowland rain forest trees in Papua New Guinea. The coccids are confined to this microhabitat but are associated with a taxonomically broad range of ants and host trees. Attendant ants belonged to six species in three genera and two subfamilies: Anonychomyrma Donisthorpe (Dolichoderinae), Crematogaster Lund (Myrmicinae) and Podomyrma F. Smith (Myrmicinae). Host plants belonged to at least five families and included both apparently specialized (with domatia) and unspecialized species. Saplings containing the nests of Anonychomyrma scrutator (F. Smith), Anonychomyrma sp. 1 and Podomyrma laevifrons F. Smith were dissected and the structure of nest chambers and their contents recorded. Only some chambers had entrance holes, but many were interconnected by transverse passages. The coccids were in low numbers and fairly evenly distributed between ant-occupied chambers. The characteristics of the Myzolecanium-ant association, the role of the coccids as trophobionts, and the nature of the plant associations are discussed. Taxonomically, new combinations are proposed by P. J. Gullan for three species previously placed in Cryptostigma Ferris: Myzolecanium endoeucalyptus (Qin & Gullan), M. magnetinsulae (Qin & Gullan), and M. robertsi (Williams & Watson).