This paper addresses the relationship between class size and student-teacher interactions as an explanation for effects of class size on achievement. Observations were conducted in kindergarten and Grade 1 classes from 46 Dutch primary schools in order to address the effect of class size on the amount and type of student-teacher interactions. The relationship between these interactions and class size was negative: more student-teacher interactions of all types occurred in smaller classes. Furthermore, the effect of smaller class size is similar for all students, regardless of their cognitive and behavioral characteristics. (Contains 4 tables.)