Our previous studies suggested that there are significant differences in the growth of the jaw and in muscle activity between breast- and bottle-fed infants. To confirm these differences quantitatively, myoelectric activities of the masseter muscles of bottle-fed babies were studied. Twelve bottle-fed babies, as well as 12 breast-fed babies as a control group, were examined electromyographically during bottle or breast feeds. The duration time of sucking bursts, interval time, cycle time, 0-to-peak amplitude, integrated amplitude of bursts, and integrated amplitude/duration time were measured and the number of bursts over 30 microV was counted. All differences of means were significant by t-test. The masseter muscle activity in bottle-fed babies is significantly reduced. Our results are contrary to previous papers in which almost the same sucking actions in both breast and bottle feeding were reported. The reason why previous researchers thought that the sucking patterns in breast- and bottle-fed babies are essentially the same is considered, and the implications of the differences for dental health are discussed.