Three isolates of human acquired immunodeficiency syndrome (AIDS) virus (lymphadenopathy-associated virus, human T cell lymphotropic virus strain III, acquired immunodeficiency syndrome-associated retrovirus) were compared with simian AIDS virus by thin section transmission electron microscopy. Simian AIDS virus was found to form intracytoplasmic A particles and to bud by both ring forms and crescent shapes. The extracellular mature simian AIDS virus exhibited a cylindrical nucleoid within a tubular or conical core shell. The simian AIDS virus was morphologically indistinguishable from Mason-Pfizer monkey virus, the prototype D type retrovirus, although the two were readily distinguished by radioimmunoassay and restriction endonuclease mapping. All three AIDS virus isolates exhibited identical ultrastructure and maturation sequences. The AIDS viruses did not form intracytoplasmic A particles and budded only with crescent-shaped nucleoids. Mature extracellular AIDS viruses contained an eccentric spherical nucleoid within a conical or tubular core shell. AIDS virus was found to show many morphologic features of the retroviral subfamily Lentivirinae. The AIDS virus isolates appear to form a new group of retroviruses.