Eukaryotic genomes are composed of isochores, . long sequences relat-ively homogeneous in GC content. In this paper, the isochore structure of Arabidopsis thalianagenome has been studied using a windowless technique based on theZcurve method and intuitive curves are drawn for all the five chromosomes. Using these curves, we can calculate the GC content at any resolution, even at the base level.