Free activities and mother-child interaction in a standard setting were observed among undernourished and well-nourished children between the ages of 7 and 18 months in West Bengal. The undernourished boys showed less vigor in play activity, less attachment behavior toward the mother, especially in interaction across a distance, and maintained close physical contact with her more frequently than did the well-nourished boys. The mothers of the undernourished boys had lower scores in four of six measures of maternal behavior. Correlations between child and maternal scores revealed a high reciprocity in mother-child interaction for the well-nourished boys, but a lack of reciprocity for the undernourished boys. The developmental quotient and activity scores of the well- and undernourished boys were not different. However, the correlations between these and the maternal scores showed a divergent pattern of relationships for the two nutritional groups.