To investigate the relationship between aggression and routine-like behaviour the response of male mice of bidirectionally selected lines for attack latency to a change in the social and non-social environment has been measured. In a non-social situation the extent of routine-like behaviour was measured in a Y-maze in which only one of the two arms gave access to the food compartments. The number of errors made in response to reversal of the arm that was blocked was taken as indicator for the degree of routine formation. Males of the short attack latency (SAL) line made significantly more errors, and hence were more routine-like in their performance, than mice of the long attack latency (LAL) line. Males of the LAL line that nevertheless had short attack latencies (i. e. aggressive LAL mice) turned out to be flexible in their behaviour; their response was similar to that of the non-aggressive LAL males. In a social situation SAL and aggressive LAL mice were used to investigate routine formation in attacking behaviour. The males were given different amounts of experience with male opponents after which their own female was introduced as opponent. The more extended the experience with male intruders was, the more SAL males subsequently attacked their female. In contrast, LAL mice appropriately changed their behaviour towards the female opponent. Thus, the attacking behaviour of SAL mice gets routine-like, whereas that of LAL males remains flexible. It is concluded that selection for attack latency generally coincides with selection for routine-like behaviour, suggesting that these two factors are influenced by many of the same genes. Regarding the fact that aggressive males of the LAL line show flexible behaviour, it may be proposed that with the phenotypic selection for attack latency there has in fact been selected for a mechanism that determines the organization (routine-like vs flexible) of behaviour.