Gamma irradiation mutagenesis is a technique commonly used to induce seedlessness in citrus fruits. We compared fruit quality traits of eight different seeded mandarin varieties within the Israeli citrus breeding collection with those of their corresponding gamma-irradiated low-seeded mutants. The mandarin varieties compared were: “Rishon” with “Kedem”; “Michal” with irradiated “Michal”; “Merav” with “Meravit”; “Vered” with “Vardit”; “Ora” with “Or”; “Murcott” with “Mor”; “Shani” with irradiated “Shani”; “King” with irradiated “King”. Mutational breeding by gamma irradiation reduced average fruit seed number by 70% - 92%. Furthermore, mutational breeding by gamma irradiation delayed ripening of the early-season mandarin varieties “Rishon” and “Michal” by 8 - 13 days, but did not delay ripening of most mid- and late-season varieties. Gamma-irradia- tion mutagenesis also reduced fruit weight of seven of the eight tested low-seeded varieties by 6 - 41 g per fruit. Gamma-irradiation mutagenesis had variable effects on biochemical composition and nutritional quality, in that some irradiated clones presented no changes whereas other had either higher or lower levels of juice total soluble solids, acidity, vitamin C and total antioxidant activity. Finally, sensory evaluations by a trained panel revealed that the flavor of most of the low-seeded, gamma-irradiated varieties was slightly or significantly preferable to that of unirradiated varieties.