Cross-suckling, i.e. when an offspring nurses a female other than their own mother, occurs in pig production systems where sows are group housed during lactation. As production systems where several sows and their litters are housed together during lactation are getting more common, the need of knowledge regarding cross-suckling increases. The overall aims with this thesis was to investigate how sow behaviour and productivity are associated with the occurrence of cross-suckling, and to investigate consistency in sow nursing behaviour related to cross-suckling. The study included information about the first 4 parities of 40 LxY sows kept in outdoor and indoor production environments. Sows were single housed the first 2 weeks after farrowing and group housed (4 or 5 sows per group) from 2 weeks after farrowing until weaning, 7 weeks after farrowing.