The backcross breeding method was applied to a co-operative tomato improvement project that involved nine research institutions in Western Canada with highly effective results. Breeding lines have been obtained from the Earlinorth × Morden BB3 cross, in which fruit size was 117.15 per cent greater and maturity equal to Earlinorth. In other lines fruit size was 44.33 per cent greater and maturity 7 days earlier than Morden BB3. Combination improvement employing correlation data for selecting the largest-fruited, earliest-ripening genotypes contributed to breeding progress. Heritability estimates indicated the selection potential of the cross. The fasciation character was found to be associated with the development of large fruit size and late maturity in certain breeding lines.