The Genetische Wissensindex (GeWi) is a German questionnaire for the assessment of general knowledge about genetics based on an American test instrument, the GKI. We modified the original instrument and constructed additional items in order to improve its psychometric properties. The scale consists of 12 items in a true-false format. So far, 578 individuals (students, patients, general population) filled in the questionnaire. An analysis of the psychometric properties reveals a satisfying distribution of item statistics and a retest-reliability of r_tt = 0,77. Associations with other scales that measure similar constructs indicate a satisfactory concurrent validity. Furthermore, the GeWi discriminates between samples characterized by previously known differences in levels of genetic knowledge, e.g. students and the general population. We recommend the scale for research and educational settings.