A deletion of 30 bases was created in the noncoding region of the human immunodeficiency virus type 1 genome that extends between the 5' splice donor and the start of the gag gene. Viral particles produced after transfection of this mutant provirus had a normal protein pattern but a reduced RNA content. The infectivity of the mutant virus was also markedly reduced but not completely abolished. Electron-microscopic examination of the mutant virions revealed major abnormalities of the nucleoid structure, mostly related to the dense material characteristic of mature particles, suggesting that the presence of RNA is essential to the normal structure of the nucleoid.