Abstract: We investigated how the behaviour of domestic foals, Equus caballus, living at pasture with their dams was associated with foal gender, mare rank and the development of abnormal oral behaviour, both during the preweaning period, and over a period of up to 4 years postweaning. A population of 186 foals belonging to private owners and commercial studs was studied. The behaviour of male and female foals hardly differed, but mare rank affected patterns of foal social interaction and suckling behaviour, with foals of subordinate mares involved in more affiliative interactions. These foals also spent more time in perisuckling activities such as teat nuzzling than foals of other mares. During the study, 18 foals developed abnormal oral behaviour before weaning and 42 foals developed abnormal oral behaviour after weaning. The development of abnormal oral behaviour was associated with suckling behaviour in a variety of ways. Foals that had already developed abnormal oral behaviour at the time of the preweaning observations were involved in more suckling terminations within bouts than normal foals or foals that developed future abnormal behaviour, and pushing the udder with the muzzle was most frequent in these foals. Foals that had no current abnormal oral behaviour, but that would develop this in the future, spent more time suckling and twice as much time teat nuzzling as other foals. The results add to the growing evidence of associations between digestive function and abnormal oral behaviour in horses.