During six blood passages of simian immunodeficiency virus SIVsm in rhesus macaques, the asymptomatic period shortened from 18 months to 1 month. To study SIVsm envelope gene (env) evolution during passage in rhesus macaques, the C1 to CD4 binding regions of multiple clones were sequenced at seroconversion and again at death. The env variation found during adaptation was almost completely confined to the variable regions. Intrasample sequence variation among clones at seroconversion was lower than the variation among clones at death. Intrasample variation among clones from a single time point as well as intersample variation decreased during the passage. In the variable regions, the mean number of intrasample nonsynonymous nucleotide substitutions decreased from the first passage (5.26 × 10² ± 0.6 × 10² per site) to the fifth passage (2.24 × 10² ± 0.4 × 10² per site), whereas in the constant regions, the mean number of intrasample nonsynonymous nucleotide substitutions differed less between the first and fifth passages (1.14 × 10² ± 0.27 × 10² and 0.80 × 10² ± 0.24 × 10² per site). Shortening of the asymptomatic period coincided with a rise in the Ks/Ka ratio (ratio between the number of synonymous [Ks] and the number of nonsynonymous [Ka] substitutions) from 1.080 in passage one to 1.428 in passage five and mimicked the difference seen in the intrahost evolution between asymptomatic and fast-progressing individuals infected with human immunodeficiency virus type 1. The distribution of nonsynonymous substitutions was biphasic, with most of the adaptation of env variable regions occurring in the first three passages. This phase, in which the symptom-free period fell to 4 months, was followed by a plateau phase of apparently reduced adaptation. Analysis of codon usage revealed decreased codon redundancy in the variable regions. Overall, the results suggested a biphasic pattern of adaptation and evolution, with extremely rapid selection in the first three passages followed by an equilibrium or stabilization of the variation between env clones at different time points in passages four to six.