The peach [Prunus persica L. (Batsch)] slow ripening (SR) trait is a mutation preventing the normal ripening process. Individuals with this phenotype are discarded in peach breeding programs. This trait is determined by a single gene (Sr/sr), where the recessive homozygote (sr/sr) confers the SR phenotype, and has been mapped to linkage group 4 of the peach genome. A large deletion of 26.6 kb containing the sequence of a NAC transcription factor has been proposed as the causal mutation. Two dominant markers based on the sequence of this region have been assayed previously and found to be diagnostic (genotypes always predicted the phenotypes). However, their dominant nature-a null allele for the marker was associated with the sr allele-made it impossible to predict the individuals that carried the SR trait. Here we used resequencing information to develop a codominant molecular marker for the SR trait in peach. The marker was validated in the 'Belbinette' 9 'Nectalady' F1 and the 'Venus' F2 populations, and in 27 lines, 18 of which are known to carry the sr allele. The marker cosegregated with the SR phenotype in all cases, allowing the discrimination of two DNA fragments of different size associated with normal-ripening alleles, in addition to a third fragment associated with the sr allele. The utility of this marker in peach breeding programs is discussed. ; Spanish Ministry of Economy and Knowledge AGL2012-40228-C02-01 Fondo de Areas Prioritarias Centro de Regulacion del Genoma 15090007 CORFO Consorcio Biofrutales 13 CTI-21520-SP03 13 CTI-21520-SP04 FONDECYT 1160584 FONDEF Genoma G13i10005 CORFO-Innova 09PMG7240