A study was made of the behavioural development of domestic piglets living in a spacious, ecologically and socially diverse environment. The effects of age and sex of piglet on the frequencies of 30 behaviour patterns over the period from birth to 14 weeks of age were examined through focal animal sampling of 35 piglets from six litters. It was found that the frequencies of lying and standing in contact with other pigs declined significantly with age while walking, wagging the tail, sniffing the ground and the biting of objects increased with age (P < 0·05). Trotting, scampering and circling with other piglets were most frequent between 2 and 6 weeks of age. The frequencies of shaking the head, ‘quacking’, rooting, shovelling, and chewing at, and shoving, other pigs also varied significantly with age. Females wagged their tails more frequently than males whereas males circled, shoved and mounted more frequently than females (P < 0·05). Results are discussed with reference to the behaviour of young pigs kept under intensive housing conditions.