The ultrastructural morphogenesis of squirrel monkey retrovirus (SMRV) and Mason-Pfizer monkey virus (MPMV) growth in cell culture were compared. Both viruses develop by a process that begins with the formation of intracytoplasmic A particles which are then enveloped at the plasma membrane during budding. SMRV also develops as a crescent-shaped nucleoid beneath a bulging plasma membrane, a development characteristic of type C oncornaviruses. Free extra-cellular mature SMRV was generally round with a centrally located electron-dense nucleoid enclosed by the viral envelope. In contrast, mature MPMV had a tubular-shaped nucleoid. Negative stained preparations of both viruses yielded head-tail forms with surface projections. By uranyl acetate/critical point drying, SMRV particles were usually round with an eccentric electron-dense nucleoid enclosed by the viral envelope, whereas MPMV particles were round and contained an electron-dense bar-shaped nucleoid. These morphological observations indicate that SMRV more closely resembles MPMV, presently the only member of genus oncornavirus type D, than other retroviruses species. However, since SMRV can be morphologically, biochemically, and immunologically distinguished from MPMV, it represents a new species within genus oncornavirus type D.