The formation of lateral roots in rice plants was examined in relation to the diameter of the crown roots on which they developed. The crown roots were classified into the followilng three types according to their acropetally decreasing rate of diameter; i.e. the crown roots of the low-(A-type), the intermetdiate- (B-type) and the high-(C-type) decreasing rates, respectively. At the lower shoot-units, the A-type roots were mostly common, while towards the upper shoot-units, the proportion of the number of each type roots varied, changing abruptly at about the IX-th shoot-unit, and above it the B-type roots and especially the C-type roots predominated. The time of crown root cmergence on the IX-th shoot-unit coincided with the panicle differentiation stage of the plant. The number oflaterals formed per unit crown root length was more in the C-type roots, decreasing successively in the B- and A-type roots. On the contrary, the length of the laterals showed a reverse tendency, being longer in the A-type roots and decreasing in the order of the B- and C-type roots, respectively. From the result mentioned above, the numerical density of laterals as well as their elongation appeared to be closely related to the changes in diameter of the crown roots ton which they were formed.