Excessive aggression between pigs at mixing is a welfare and production issue resulting in stress, injuries and economic losses. If it can be demonstrated that aggression is a consistent behaviour trait, it might provide a means to reduce these losses. To test for consistency of aggressiveness, 163 male and female damline pigs, mixed at weaning (mean ± SD day = 33.7 ± 3.4), were given resident-intruder tests (RITs) on successive days (test pairs a and b) on up to three occasions: day 60 (test 1); day 95 (test 2); and day 130 (test 3). Pigs tested at all three time points (T123 n = 90) showed consistency in occurrence of attacking within (a vs. b) and between tests (1 vs. 2, 2 vs. 3 and 1 vs. 3), suggesting that aggressiveness is a moderately stable temperament trait. Compared to previous studies of this kind there were clear differences between male and female pigs, with a higher than expected rate of mounting in male pigs. Mounting also appeared to be a consistent trait, beginning from a surprisingly early age (day 60). Latency to attack decreased in the female pigs (n = 53) after test 1a, but did not change after this, although a further unexpected finding, was an effect of boar line on the occurrence of attacking in female pigs. In the male pigs (n = 37) aggression reduced over time, which we propose was primarily as a result of increased mounting. Sex differences in aggression and mounting were also seen when the behaviour of pigs naive to the RIT were first tested (test 1, treatments T123, n = 90 and T13, n = 22; test 2 T23, n = 24; test 3, T3, n = 27), suggesting they result from age rather than experience. The effect of prior RIT experience on aggression in the final test (test 3) was examined in terms of: (1) the amount of experience, by comparing pigs with no (T3), one (T23 and T13) or two (T123) previous experiences of the RIT; and (2) the age at first experience, by comparing pigs first tested in test 1 (T13) or test 2 (T23). Prior RIT experience made little difference to the level of aggression (attack latency), although more experienced pigs were more likely to attack than to mount. In conclusion, pigs showed stable individual differences in both aggression and mounting, which were affected more by sex and age than by prior test experience.