This study explored the relation between mother-infant coordinated interpersonal timing, an automated microanalytic measure of dyadic vocal coordination, and maternal sensitivity. Thirty-four mothers and their developmentally normal 4-month-old infants were audio recorded during a 20-minute laboratory vocal interaction session, which was later analyzed for coordinated interpersonal timing. Maternal sensitivity ratings were based on a videotaped 45-minute unstructured interaction period also obtained during the laboratory visit. The findings indicated a significant curvilinear relation between the degree to which mother coordinated her noninterruptive co-occurring speech to that of her infant (r=.381) and revealed that mothers highest in sensitivity were characterized by moderate levels of coordination. The finding supported the notion that examining mother-infant interaction at the specific behavioral level, while incorporating tests of nonlinear trends, may provide important information about the nature of sensitive parenting. (Contains 40 references.) (Author/AMC)