Objective: To characterize the envelope (Env) glycoprotein of HIV-1 in mother-infant pairs (MIP) that underwent near simultaneous or acute-phase seroconversion, we examined the Env sequence of the transmitted viruses and compare viral evolution within the pair. Design: Three MIP from a Zambian cohort that seroconverted at the same sampling time were identified and followed longitudinally. Methods: The V1-V5 region of the HIV-1 Env gene was sequenced for each sample collected. Phylogenetic and population genetics analyses were carried out to subtype the viruses, estimate relationships among viral genotypes, and compare molecular evolution between the viral populations. Results: Genetic analyses demonstrated a close intrapair relationship between viral sequences from each MIP. Transmission involved several closely related viral genotypes and did not result in a reduction in viral diversity. Amino acid changes were not evenly distributed along Env V1-V5 but concentrated in concordant areas within each MIP. Several positions under positive selection were shared between the MIP viruses. Interestingly, selective pressure on the virus was higher in the infants than in the mothers.