The aim of this study is to examine how librarians and preschool teachers collaborate while using a bookmobile. It also examines what impact the cooperation has on the children’s literacy since it’s in both their interests to promote children’s language development. The study seeks to answer the following questions: How do the librarians and the preschool teachers collaborate using the bookmobile? How do the preschool uses the bookmobile? In which way do the cooperation between the librarians and teachers impact on the children’s languages development? The study is based on qualitative interviews with two librarians working on the bookmobile and three preschool teachers. The theoretical framework that is used in the study is a seven-grade taxonomy written by Ögland, Lundgren and Wockatz. The taxonomy is used to evaluate the cooperation between the librarians and the teachers. The study shows that the librarians bring books for special theme works and media that the preschool teacher is asking for. It also shows that the preschool only use the mobile library to borrow and return books. The librarians and the preschool teacher are working with children’s literacy in opposite directions and if they don’t start collaborate within this part it can affect the children’s reading in a negative way.