||A 3-year study was carried out on the developmental behavior of foals from birth to 24 weeks of age and the behavior of mares living with foals. Mutual-grooming partners of foals were primarily other foals. The peak frequency of mutual grooming occurred during Weeks 9-12, when fillies mutual-groomed 1.6 times h-1 and colts mutual-groomed 0.9 times h-1. Fillies mutual-groomed more frequently than colts (P < 0.025). Fillies mutual-groomed randomly with colts and other fillies (P < 0.05), whereas colts mutual-groomed almost exclusively with fillies (P = 0.03). At all ages studied, if a foal's nearest neighbor was not its mother, it was more likely to be another foal than would be expected if the foal was associating randomly with non-mother ponies. Fillies were more likely than expected to have a filly rather than a colt as their nearest neighbor (P = 0.01). Thus, during their first few months of life, the foals studied exhibited patterns of behavior which were consistent with the development of the usual social milieu of unmanaged adults, in which several mares form a cohesive herd with one or more stallions associating with them.