Forty-eight pigs were fostered at 2 to 9 h or at 2, 4, or 7 d of age to determine age-related differences in the behavioral interactions between fostered pigs, resident pigs, and the sow during the first 6 h after fostering. Pigs were fostered in pairs to sows with litters of comparable age. In each test, the two fostered pigs and two resident pigs of similar body weight were observed for 6 h, and rate of ambulation, successful and unsuccessful suckling attempts, vocalizations, and approaches to within a body length of the sow's snout were recorded. Sniffs and aggression directed at the fostered and resident pigs by the sow were also recorded. Eleven of 12 pigs fostered at 2 to 9 h of age met the criterion for successful suckling within 6 h of fostering. Suckling success at the three older ages (2, 4, and 7 d) was much lower (25 to 50%; P < .005) after 6 h had passed. The behavior of pigs fostered at 2 to 9 h was similar to that of the resident pigs at all ages, whereas the pigs fostered at 2, 4, or 7 d showed a greater reluctance to engage in suckling, higher rates of ambulation, and more frequent vocalizations. Sows tended to be more aggressive toward older fostered pigs. It was concluded that fostering pigs older than 1 to 2 d of age will slow the rate at which they integrate into the new sow-litter environment and engage in suckling behaviors.