The application of megabase methods is essential for positional cloning of genes for which no products have been characterized. We have set about the isolation of genes conferring resistance to pathogens in the french bean Phaseolus vulgaris. We describe here the isolation of high-molecular-weight DNA, pulse-field gel electrophoresis and large-scale mapping of the Pal2 locus in P. vulgaris. We describe the construction of such a map by embedding DNA isolated from leaf nuclei in agarose piugs, restricting it with rare cutters and subjecting the DNA to pulse-field gel electrophoresis.