Sustainable animal production should take animal welfare into account. How animal welfare is incorporated into farming practices will, however, largely depend on the farmer’s choices. These choices may depend on the farmer’s perception of animal welfare and the willingness to change the current situation. Aggression between group housed pigs is a longstanding welfare issue and research efforts have resulted in little to no change in practice. Our objective was to gain insight into farmers’ perception of aggression between growing pigs and their opinion about a method shown by research to reduce the expression of this behaviour. Pig farmers in the UK were asked about their management and perception regarding aggression through a postal survey. Respondents (n=167) had a breeder-tofinisher farm (n=114; 585±123 sows; range 0-7000), breeder-weaner farm (n=10; 718±433 sows; range 15-45000), or grower/finisher farm (n=32; 1291±187 grower/finishers; range 24-5000). The majority of the respondents did not consider aggression at mixing as a problem which needed to be addressed (73% at weaning). For mixing at the finisher stage, 43% did consider aggression a problem and indicated they would consider a solution if available. Farmers who considered aggression at the grower/finisher facilities a problem were on average younger (55±12 years) than farmers who did not consider it a problem (61±12 years; P=0.02). When respondents ranked welfare issues on what they found most important to reduce at the grower/finisher phase, they ranked tail/ear/flank biting as more important than mounting and lameness, but not different from aggression. Currently, 27% of the pig breeders (respondents keeping sows) applied co-mingling of piglets (i.e. ‘socialization’), in which piglets are pre-weaning introduced to piglets of other litters to reduce aggression later on, and 22% 3 had worked with such a strategy in the past. Farmers applying co-mingling did not differ in their perception of aggression compared to farmers who did not co-mingle. Respondents expressed concerns about co-mingling with regard to practical management (48%), aggression of the sow towards piglets (33%), reduced piglet growth rate (24%), fights between piglets (22%), cross-suckling (20%), missed suckling bouts (16%), and stress for the animals (16%). Half of the breeders were in favour of co-mingling (51%) whereas the other half was against (49%). As part of a sustainable approach integrating all stakeholders, knowledge of farmers’ perception of aggression may help align research questions in this area with the concerns of the stakeholders.