The organoleptic quality of fleshy fruits is in a large part defined by their composition of soluble sugars and organic acids. An F 2 population issuing from a cross between two peach varieties, 'Ferjalou Jalousia', a non-acid peach, and 'Fantasia', an acid nectarine, was analysed over 2 successive years for agronomic characters and for molecular-marker (isoenzymes, RFLPs, RAPDs, IMAs and AFLPs) segregations. Blooming and maturity dates, as well as productivity, were noted for each tree. Four fruits per tree were analysed at maturity for fresh weight, colour, pH, titratable acidity, soluble-solids content (SSC), acid (malic, citric and quinic acids) and sugar (sucrose, glu- cose, fructose, sorbitol) contents. QTLs were detected for all fruit components analysed, except for fruit col- our. The QTLs for nearly all components were present on two linkage groups. For productivity, fresh weight, pH, quinic acid, sucrose and sorbitol content, all the detected QTLs displayed the same e⁄ect as the parental phenotypes. By contrast, for maturity date, titratable acidity, malic and citric acids and fructose, some QTLs displayed the same e⁄ect as the parental phenotypes while others displayed the opposite e⁄ect. The fraction of the total variation in each trait throughout the population explained by the QTLs was very high and reached more than 90% for some characters. For most