A rice lateral rootless mutant k209 with high temperature sensitivity was isolated from an ethylmethane sulfonate (EMS)mutagenized rice library. After growing under normal (day 32 ℃/ night 22 ℃) and high (day and night 34 ℃) temperature for 7 days, phenotype analysis and lateral root primordia examination by methylene blue staining were conducted. The F₂ populations from crossing of k209 with wild type (WT) Kasalath and Nipponbare were used for genetic analysis and gene mapping, respectively. Results showed that under normal temperature there was no significant difference in the plant height, primary root length and adventitious root length between k209 and WT, but the length and number of lateral roots of k209 were decreased. However, under high temperature, k209 showed significantly shorter shoots, primary roots and adventitious roots and no lateral root. Furthermore, methylene blue staining analysis revealed that normal lateral root primordia could form in the mutant under both normal and high temperature. Under high temperature the number of lateral root of k209 was only 58.03% of the WT, and they could not outgrowth through the epidermis. Genetic analysis showed that the mutant phenotype was controlled by a single pair of recessive nuclear gene. Mapbased cloning analysis located the gene to a 4 002 kb region between InDel markers 7522K and 11524K on chromosome 4. This study will help the cloning of this mutant gene as well as further elucidation of the molecular genetic mechanism of lateral root development in rice.