Quantitative analysis and developmental observations on the planum temporale of 157 fetus and infantal brains have been made. The results are as follows: 1) left planum temporale of 82％ of the fetal brains and 76％ of the infantal brains are larger than the right. 2) Average area values of the left planum temporale are signifcantly larger than the right. 3) The earliest appearance of the planum temporale is in the sixth gestational month. 4) Rate of appearance of the left planum tempo-rale in fetuses is higher than that of the right. 5) Right/left ratio of planum temporale increases with the development of the embryo. These findings indicate that the asymmetry of the left and right planum temporale is present since the appearance of the planum temporale, and changes as the fetus develops.