Tetraploid somatic hybrids, derived from ten different fusion combinations, were grown in the field to study the inheritance of quantitatively-controlled agronomic traits. The data from two seasons showed that the heterosis in yield varied between 70% and 230% compared to the mid-parent value of the dihaploid fusion clones. The relative yield of the hybrids compared to two standard varieties (= 100%) ranged from 47% to 106%. Despite large differences in yield between the single hybrids of the same fusion combination, no significant differences could be detected between the hybrids. The analysis of the yield components revealed that the heterosis was mainly due to a highly increased tuber weight, whereas the tuber number was in general intermediate to that of the parent clones. In three combinations, the starch content of the hybrids was 15-20% higher than the midparent value and in the others it was intermediate. Fusion of clones with long oval tubers and with round or round-oval ones showed the round/round-oval form was dominant. Fusion combinations of tuber flesh colour indicated that in most cases yellow was dominant to white, while red skin colour was dominant over yellow.