In the present longitudinal study, we investigated attachment quality in Portuguese mother-infant and in father-infant dyads, and evaluated whether attachment quality was related to parental sensitivity during parent-infant social interaction or to the amount of time each parent spent with the infant during play and in routine caregiving activities (e.g., feeding, bathing, play). The sample consisted of 82 healthy full-term infants (30 girls, 53 boys, 48 first born), and their mothers and fathers from mostly middle-class households. To assess parental sensitivity, mothers and fathers were independently observed during free play interactions with their infants when infants were 9 and 15 months old. The videotaped interactions were scored by masked coders using the Crittenden's CARE-Index. When infants were 12 and 18 months old, mother-infant and father-infant dyads were videotaped during an adaptation of Ainsworth's Strange Situation. Parents also described their level of involvement in infant caregiving activities using a Portuguese version of the McBride and Mills Parent Responsibility Scale. Mothers were rated as being more sensitive than fathers during parent-infant free play at both 9 and 15 months. There also was a higher prevalence of secure attachment in mother-infant versus father-infant dyads at both 12 and 18 months. Attachment security was predicted by the amount of time mothers and fathers were involved in caregiving and play with the infant, and with parents' behavior during parent-infant free play.