In this article a study is presented of selective affiliation of two- and three-year-old children in the natural situation of Dutch daycare centers. Data of (behavioral) characteristics of 674 pairs of children (dyads) were collected. Multi-level analyses of these dyadic data were conducted and showed that the chance of friendship between two children is related to similarity in age and gender. No friendship preference for similarity in cultural background was found. The friendship preference for similarity in gender and age seems to be related to a preference for specific play activities. In all groups except one, a relation between friendship and familiarity was found. Part-time attendance of the young children at the daycare groups caused group instability and probably restricts the chance to develop friendship relations. Children's tendency to be attracted to similar children raises the pedagogical issue of diversity in group daycare. (Contains 4 tables and 1 note.)