A mutation of a gene (discovered in Festuca pratensis Huds. and designated sidy) confers indefinite greenness on senescing leaves. Via intergeneric hybrids with Lolium multiflorum L. and Lolium perenne L., the mutant gene (sidy) has been introgressed into a range of Lolium backgrounds. Using genomic in situ hybridization we have identified segments carrying sidy in recombinant chromosomes of Lolium–Festuca introgression lines. We also used L. perenne lines segregating 1:1 for the staygreen character to tag the gene with molecular markers. In two mapping populations a total of 84 genotypes were screened with isoenzymes, RAPD primers, RFLP probes and AFLP primer pairs. Over 180 polymorphic loci were identified, representing 10 linkage groups spanning 600 cM. Two AFLP markers are linked to sid at 4.6 and 14.9 cM, close enough to be usable for marker-assisted selection. Introgression of sidy into Lolium temulentum L. resulted in the production of near-isogenic inbreeding lines suitable for comparative studies of gene expression. Using a variation of the method of representational difference analysis a very small number of cDNAs have been identified as promising candidates for sid, or genes directly regulated by sid.