SUMMARYThe field occurrence of symptoms of beet western yellows virus (BWYV) disease in breeding lines of summer lettuce was studied in trials conducted from 1971 to 1974. No evidence of immunity was found in seventy cultivars and over 500 lines from ten crosses, but some lines, notably those with Little Gem as one parent, appeared to be less affected by the disease.When the number of plants showing symptoms was recorded on a weekly basis it was apparent that only minor differences existed in the response to infection. In the earlier maturing lines, 50 % of the plants exhibited symptoms within 16 to 20 days of hearting, and later maturing lines reached the same level of disease incidence within 7 to 16 days after hearting.The search for resistance should be widened to include wild species; strict selection techniques up to flowering time should also be used to increase field resistance in the present cultivars.