This study reports on a new false belief measure in a sample of 124 children and adolescents with or without high functioning autism (HFASD). In the classic paradigm, a participant predicts in which of two discrete locations a deceived protagonist will look for an object. In the current Sandbox task, the object is buried and reburied in a sandbox, thus creating a continuum between locations. Compared to typically developing individuals (n=62), those with HFASD (n=62) showed a larger egocentric bias on the Sandbox task. They failed to take the protagonist's false belief into account, despite their adequate ability to infer advanced mental states. This indicates that sensitive measures can reveal subtle first order Theory of Mind impairments in HFASD individuals.