Social preferences of familiar over unfamiliar social stimuli in chicks may be used to measure sociality, a characteristic important for the welfare of poultry in commercial production. We studied social preferences and reaction to strangers in young White Leghorns and red jungle fowl (Gallus gallus) in 3 tests. All chicks were raised and housed in 2 groups of 34 individuals per breed. At 24 to 29 d of age 38 chicks per breed were tested in 2 runway tests. In the first, chicks had a free choice between familiar and unfamiliar breed members, and in the second the choice was between unfamiliar chicks of their own breed and the other breed. On d 41 to 42, spacing and agonistic interactions of 28 pairs of chicks per breed (in half of the pairs, chicks were unfamiliar to each other) were observed in an open field for 10 min (pair test). In the first runway test, clear preference for familiar chicks and avoidance of unfamiliar social stimuli was found only in Leghorns, whereas both breeds showed a preference for their own breed members in the second runway test. Affiliation to the familiar breed, however, was more pronounced in Leghorns. In the pair test, Leghorns were significantly more involved in agonistic interactions than wild-type chicks. Avoidance of unfamiliar and preference for familiar conspecifics might suggest a weaker capacity of Leghorns to cope with novel social and environmental stimuli, which might have implications for the welfare of the birds in production environments when encountering unfamiliar individuals.