To investigate the influence of interpersonal distance on vocal activity in the mother-infant dyad, behaviors of 24 mothers and their three-month-old infants were continuously recorded in the home across four naturally-occurring maternal proximity conditions. Although no difference in infant vocal activity was found between mother-present and mother-absent conditions, infants spent more time vocalizing during maternal absence than while being held. Infant vocalizations when mothers were absent, especially for females, were frequently directed to objects. Vocal activity of infants, but not that of mothers, was differentially affected by the two closest proximity conditions. Infants vocalized significantly more of the time that they were within arms reach of their mothers than while they were being held. The results suggest that proximity is an important contextual factor for the analysis and interpretation of mother-infant interaction data.