This 6-wave longitudinal study of 161 German 5- to 10-year-olds (89 females, 72 males) found that scientific-reasoning abilities first develop at 6 years, with significant progression between all waves. Abilities were highly stable, with the kindergarten score predicting 25% of end-of- elementary-school variance. Individual but not developmental differences were related to language abilities (.39), mindreading skills (.33), and the parental education level (.36). In early elementary school, basic mindreading skills predicted scientific reasoning (.15), but not vice versa; in late elementary school, bidirectional associations emerged between scientific-reasoning and more- complex mindreading (.11-.33). Our findings suggest that mindreading is a precursor for the development of scientific reasoning, and that older children use scientific reasoning to test and revise their advanced theories of mind.