-Predicting that an outbreak of tail biting is going to occur in a group of pigs would be a useful tool for farmers. In a prospective longitudinal study, 24 groups of c.30 undocked pigs were followed from birth to slaughter weight. Four groups had to be excluded from the analysis, the remaining groups were classified as having No Outbreak (n=6), Underlying Outbreak (n=8) or Severe Outbreak (n=6) of tail biting. The hypotheses examined were that pigs would be more active, perform more tail-orientated behaviours, or have their tails tucked under their body more in groups that went on to have outbreaks than in those that did not. Direct observations were made at 7, 11, 15 and 19 weeks of age and video recordings were examined for the 4 days prior to an outbreak. All outbreaks occurred after the behavioural observation at 2 11 weeks of age. Activity levels were significantly higher in Severe Outbreak groups in the four days prior to an outbreak, with more pigs Standing (p<0.05) and significantly fewer pigs Sitting (p<0.05) or Lying Inactive (p<0.05) than in matched control groups. Comparisons of the Severe, Underlying and No Outbreak groups at 7 and 11 weeks of age, prior to any outbreaks, showed no difference in activity levels. However, at these ages, levels of Tail