C3H/He mouse embryos in utero were exposed to intensepulsed ultrasound on the 8th day of gestation (VP day = 0). Fetal anomaly was observed in mice exposed for 5 min to 58.6 W/cm2 peak intensity pulsed ultrasound, but not in those of a group exposed to a reduced peak intensity having the same pulse width and repetition frequency. In spite of similar average acoustic intensities, the group exposed to the higher intensity (59.4 W/cm2) exhibited fetal anomaly. There was no fetal anomaly in the lower peak intensity group. No increase in fetal anomaly was observed after exposure to ultrasound having a smaller pulse width (3 .mu.s), even though the peak intensity was 56.3 W/cm2. It is suggested that teratogenecity may depend on both the peak intensity and the pulse width of the ultrasound. Critical acoustic intensity on the 8th day of pregnancy in mice is estimated to be 60 W/cm2 SATP and 1.2 W/cm2 SPTA values.