Aggression seen on mixing of young pigs is a major welfare concern in modern farming. One solution may be to reduce the likelihood of aggression within a group by using combinations of individuals that facilitate rapid integration. This idea has been investigated by Mendl and Erhard (1997) using pigs with various levels of aggressiveness. An attack latency test was used to measure individual aggressiveness. Experimental pigs were placed in a familiar pen with an unfamiliar pig and the time taken to attack (i.e. attempt to fight) the intruder measured. In a substantial number of tests (e.g. 61% in the present study) no attack occurs in the time allowed (5 minutes) These pigs vary in their interactions with the intruder, ranging from no interest at all to persistent interest with isolated aggressive acts but no escalation to fighting (personal observation). The aim of this study was to investigate social behaviour throughout the test, thereby allowing a more detailed analysis of aggressiveness to be made.