Background: After-school programs (ASP) provide opportunities to influence children's physical activity (PA). However, little is known about the PA levels of the youngest schoolchildren. The aim of the study was to describe the level of PA intensity and sedentary time among first graders attending ASP in Norway, and to investigate factors that are associated with PA in the ASP. Methods: First graders from 14 ASPs were invited to participate in the study and 426 children were included. The children wore an accelerometer for five consecutive days during their stay in the ASP. Light, moderate, and vigorous PA and sedentary time were registered and analyzed. Results: On average, the children accumulated 25.8 minutes of moderate and vigorous physical activity (MVPA) during their ASP-stay. However, there was considerable variation within the sample. Sex, as well as body mass, were associated with MVPA. The activity level increased substantially when the children had to be outside. Conclusions: The findings show that on average, the children were quite physically active in the ASP. However, it seems critical to devote increased attention to the least active children and to include extensive outdoor play-time in the ASP schedule.