Longitudinal surveys of commercial Pekin ducks from four farms with a history of poor flock performance and high mortality rate showed that there was lymphoid depletion of the bursa of Fabricius in most ducks selected from 7 days of age onward. Such lesions were not present in Pekin ducks from a farm with a history of good flock performance and low mortality rate. These findings suggested an association between poor performance of flocks and the presence of lesions in the bursa of Fabricius. The prevalence of severe bursal depletion was found to be similar to the mortality rate up to 49 days of age. As most mortalities were due to septicaemia, it seems possible that septicaemic infections were developing secondary to immuno-suppression associated with bursal depletion, as occurs in infectious bursal disease (IBD) of the chicken. The disease was transmitted to day-old Pekin ducklings by inoculation with bursal homogenate, but not to day-old chickens. The causal agent was shown to be less than 220 nm in diameter and did not appear to be IBD virus. Its nature remains to be determined.