The preference of the infant for mother versus father as an attachment figure and as a secure base for exploration is examined in this study. Subjects were fifteen 15-month-old infants whose mothers were their primary caregiver. The Ainsworth Strange Situation procedure was modified to permit both parents' simultaneous presence during separation and reunion. Ainsworth's qualitative ratings of attachment and frequency measures of exploration were used. Results indicate that separation from mother inhibited exploration more significantly than separation from father and that mother was preferred to father on proximal attachment measures at reunion. The importance of viewing the attachment-exploration balance as an index of the affective and cognitive aspects of the child's developing personality is stressed, and implications for day care and academic achievement are mentioned. (Author/JA)