Tos17 is a Ty1-copia retrotransposon existing in low copy number and known to be specifically activated during tissue culture in cultivated rice: Tos17 has been further used for the insertional mutagenesis of this model cereal plant (Hirochika 2001). As a first attempt to understand variation of activation among native copies and among rice cultivars, we have localized Tos17 copies in the indica cultivars IR64 et Kasalath and in the japonica cultivars Azucena and Nipponbare though mapping in two recombinant inbred line (RIL) populations derived from the F1 hybrids IR64/Azucena and Nipponbare/Kasalath. A preliminary analysis with methylation -sensitive restriction enzymes demonstrated that most of the Tos17 copies are heavily methylated in these cultivars. Localization of the native copies of the 4 cultivars unexpectedly showed that most of the copies were assigned to or close to chromosomal regions known to contain QTLs for tissue culture ability in rice. In order to understand relationships between active copies, inactive copies and tissue culture ability we have evaluated the amenability to tissue culture of recombinant lines derived from IR64/Azucena and Nipponbare/Kasalath selected for containing various combinations of the parental native copies, along with the parental lines, used as control. In IR64/Azucena RILs, a relationship was established between the regeneration potential of the lines and presence of chromosomal regions contributed by each parent and known to bear QTLs for callus induction or plant regeneration abilities in rice. No newly inserted Tos17 copy was detected among the primary regenerants of the lines analyzed, whereas 55% of the regenerants of the parental control Azucena harboured an additional copy, suggesting that the active Tos17 copy of Azucena is inactive in RILs. Results of comparable study conducted with Nipponbare/Kasa lath RILs will be presented as well. This work was funded by the French National Genomics initiative Génoplante. (