Using an improved rice root research system,crown root numbers in 42 cultivated rice were analyzed.Results showed that different cultivars have different crown root numbers,ranging from 2 to 12 at average.Two cultivars with extremely few crown roots and 2 cultivars with large numbers of crown roots were selected for further analysis with the polar auxin transport inhibitor NPA.By localized application of NPA at the shoot-root junction of the 4 selected cultivars,we found that NPA appeared to reduce crown root numbers in general.Cultivars with large number of crown roots were more sensitive to low concentration of NPA than cultivars with small numbers.These data indicated that auxin and polar auxin transport determines crown root number in rice.