Candidate male and female breeders from nine genetic lines of turkeys that were reared intermingled, with the sexes housed in different buildings on the same farm, were vaccinated with a live Newcastle disease virus vaccine (B1 type, LaSota) just prior to the commencement of egg production. In 1999, an average mortality for all lines of 5.8% occurred during the 10 days immediately following vaccination and the level of mortality varied among lines. Mortality was, in general, greater in large-bodied lines than in small-bodied lines. Affected birds exhibited gross multiple areas of focal necrosis in the liver and spleen and congestion of the heart and lungs. The percentage mortality occurring following similar vaccination in 2000 averaged 2.6 for the 10 days following vaccination and mortality was greater (P ≤ 0.05) in one line (F line) than the other genetic groups and higher in females than in males. Mortality in the F line, selected for increased body weight and known to be susceptible to various diseases, averaged 15.1% for both years. Attempts failed in both years to isolate Pasteurella multocida or other bacteria. There was a positive correlation between increased body weight and increased mortality following vaccination with the live LaSota vaccine.