In this study, the concept of 'goodness-of-fit' between the child's temperament and the environment, introduced by Thomas and Chess [Temperament and Development, Brunner/Mazel, New York, 1977], is applied within the setting of center day care. Mothers and primary professional caregivers of 186 children, aged 6-30 months, participated in this study. The child's problem behaviors were assessed with the CBCL Teacher Report Form [Achenbach, T.M., Guide for the Caregiver-Teacher Report Form for Ages 2-5, Department of Psychiatry, University of Vermont, Burlington, VT, 1997]. The child's socio-emotional well-being in day care was measured with the Leiden Inventory for the Child's Well-Being in Day Care. The Infant Characteristics Questionnaire measured the child's temperament. Children with an easier temperament showed less internalizing and total problem behavior and more well-being. The results suggest that for children with a more difficult temperament, several parallel care arrangements interfere with the process of adapting to the day care setting. Also, our results indicate that in the group of children with greater availability of trusted caregivers, a more easy-going temperament was associated with more well-being. The association between temperament and well-being was not found in the group of children with less access to trusted caregivers. © 2004 Elsevier Inc. All rights reserved.