Mason-Pfizer monkey virus (M-PMV) is a primate retrovirus that shows type D morphogenesis in mammalian cells. Immature intracytoplasmic A type particles (ICAPs) preassemble in the infected cell cytoplasm migrate to the plasma membrane and are released by budding. This is in contrast to retroviruses that show type C morphogenesis, where assembly and budding occur concurrently at the plasma membrane. We expressed the M-PMV structural genes (gag-pro-pol) in insect cells using a recombinant baculovirus. The polyprotein precursors assembled predominantly intracellularly, although a small proportion also assembled at the membrane. The protease enzyme was active since mature particles were identified in the culture supernatant. We also expressed the M-PMV mutants, D26N and gag-STOP, which carry a nonfunctional protease or fail to express the protease gene, respectively. These baculovirus recombinants generated a homogeneous population of immature M-PMV capsids having exclusively type D morphogenesis. Sufficient quantities of polyprotein precursors were synthesized to be visualized directly on a Coomassie-stained protein gel, and the capsids were subject to purification. These results provide the first expression of type D retrovirus particles using the baculovirus expression system.