Observations in an infant day care program in which mothers take part indicated "jealousy" in infants as young as 5 months when they saw their mother or even the assigned caregiver paying attention to another infant. A total of 18 first-born females between 11 and 13 months of age were the subjects in a systematic study of this phenomenon. Infants were videotaped through a one-way mirror under four conditions: baseline, mother-mother interaction, mother-other baby interaction, and mother-baby interaction. Behavior of the experimental infant was coded using a time sampling technique with 43 mutually exclusive events. Intercoder reliability was 92.4. An analysis of variance for repeated measures revealed significant differences. When infants observed their own mother play with another infant of the same age, exploratory and interactive toy behavior decreased. Unexpectedly, proximity-seeking and orienting to mother also decreased. Practical and theoretical implications of the findings are discussed. (Author/RH)