The production of high quality fruit is the main aim of fruit growers. In areas where summers are warm, high apple quality is often difficult to attain, particularly with respect to red cultivars; colour is one of the main features affecting fruit quality. In vigorous red apple cultivars, the use of very dwarfing rootstocks might help overcome the lack of colouring in the fruit. In experiments performed in 2003 and 2004, specimens of apple cv. Jonagored grafted onto P16 rootstocks were compared to others grafted onto M9 rootstocks; fruit thinning was performed to various extents so that trees carried different crop loads. Under the warm summer conditions of the Ebro Valley, Spain, the P16 rootstock showed very strong dwarfing properties. "Jonagored" trees grafted onto these rootstocks had shorter shoot lengths and a smaller leaf area than those grafted onto M9 rootstocks. The fruits of "Jonagored" apple trees are usually large, but in 2004, when the crop loads of the P16-grafted trees were far greater than those of the M9-grafted trees, the fruits of the former were smaller than those produced by the latter, although still commercially suitable. The rootstocks were found to affect several fruit quality variables. The soluble solids content of the fruits of the P16-grafted trees was greater than that of the M9-grafted trees, and negatively correlated with the crop load. In 2003, when summer temperatures were higher, fruits developed a less intense red colour than in 2004, although those from the P16-grafted trees were redder in both years. This difference in fruit colour was caused by the higher anthocyanin and lower chlorophyll concentrations of the skins.