In this study, children's attachment relationships with their professional caregivers in center day care were observed for 48 children. We explored whether more positive caregiving was associated with a more secure attachment relationship and whether this association was stronger for more temperamentally irritable children compared to less irritable children. Trained observers coded the attachment relationship in the day care setting using the attachment Q-sort. The observational record of the caregiving environment was used to assess children's individual experience of positive caregiver-child interaction in the classroom. When caregivers showed more frequent positive caregiving behavior, children showed more secure attachment behavior toward their primary professional caregiver. Temperament was not related to attachment security, nor did it serve as a moderator. Consequently, no support for Belsky's susceptibility hypothesis was found. © Blackwell Publishing Ltd. 2008.