Abstract The vocalizations and activity of suckling piglets aged 4 to 25 days were studied during 5-min periods of social isolation in an unfamiliar enclosure. The animals became very active and vocalized repeatedly during the test, but the reaction was attenuated among older litters. Calls were predominantly closed mouth grunts at low rates of calling, and mainly squeals and open mouth grunts at high overall rates. Squealing correlated with open mouth grunting and with the activity measure both within and between litters. Even so, the measures appeared to be affected independently by the piglets' behaviour in the home pen just before the test, and by their habits of fighting during suckling. Individual scores for squealing and open mouth grunting correlated with scores for the same measures on a subsequent re-test. Relationships among the types of vocalization, and individual differences among piglets in their reactions to the test, were discussed.