One hundred 12 month old Merino wether sheep from a flock with longstanding ovine paratuberculosis were studied until they were 22 months old. Serum samples and faecal samples were collected every 6 weeks for 10 months and liveweights were recorded. Fleece weight and average fibre diameter were recorded at 19 months of age. Serum samples were tested with an ELISA. At the beginning of the experiment, no sheep had ELISA OD ratios greater than 2.4 but by 22 months 18 sheep had ratios greater than 2.4 or had had on at least one occasion. These 18 sheep were killed and necropsied and paratuberculosis status was determined by histopathology. 6 sheep were found to be affected. Retrospective culture of the faeces from these sheep was used to determine when faecal shedding commenced. There was no effect of sub-clinical infection on wool production but an effect on liveweight was evident from about the time when serology was positive.