Analysis of the phenotype of the molecular clones of cytopathic and fusogenic, noncytopathic and nonfusogenic, and chimeric proviruses of human immunodeficiency virus type 2 (HIV-2) suggests that the major determinant of the attenuated fusogenicity and cytopathicity of HIV-2 was located in the 3‘-half of the genome, with envelope playing the more dominant role. However, no single linear domain within the envelope, including the major CD4 binding domain and fusogenic domain, was sufficient by itself for syncytia induction and cytopathic effects. Truncation of the transmembrane envelope glycoprotein downstream of the transmembrane region was not a major factor in this regard. However, truncation within the transmembrane region rendered the provirus replication incompetent. The regulatory genes (tat, rev) and auxiliary gene (nef) did not seem to play a critical role in determining HIV-2 fusogenicity in vitro. The results suggest the importance of the overall conformation of the envelope in the divergent phenotypes of HIV-2.