We reported previously on a vaccine approach that conferred apparent sterilizing immunity to SIVsmE660. The vaccine regimen employed a prime-boost using vectors based on recombinant vesicular stomatitis virus (VSV) and an alphavirus replicon expressing either SIV Gag or SIV Env. In the current study， we tested the ability of vectors expressing only the SIVsmE660 Env protein to protect macaques against the same high-dose mucosal challenge. Animals developed neutralizing antibody levels comparable to or greater than seen in the previous vaccine study. When the vaccinated animals were challenged with the same high-dose of SIVsmE660， all became infected. While average peak viral loads in animals were slightly lower than those of previous controls， the viral set points were not significantly different. These data indicate that Gag， or the combination of Gag and Env are required for the generation of apparent sterilizing immunity to the SIVsmE660 challenge.