Syncytium-inducing (SI) type 1 human immunodeficiency viruses (HIV-1) replicate faster in vitro and are generally more cytopathic to T cells than non-syncytium-inducing (NSI) HIV-1. Early in infection, the virus population typically consists of NSI viruses, with SI viruses appearing later. This is true even when both SI and NSI viruses are transmitted, and when SI viruses dominate the virus population peak seen during primary infection. Here, Phillips's model of HIV dynamics during primary infection (Science 271:497-499) is modified to map the growth trajectories of SI and NSI subpopulations. The model predicts that with certain rate constants, SI viruses may show a more precipitous decline in their numbers during primary infection than NSI viruses, and this may account for the observed dominance of NSI viruses early in infection.