This research attempts to measure personality by monitoring behaviour in a virtual environment. A computer game was created to measure a trait of the Five Factor Model of personality: extraversion. Test-items were created to measure extraversion and its facets, as specified by Costa and McCrae [7]. For this purpose, 25 items were built into the virtual environment. In order to test if these measures actually measure extraversion and its facets, an experiment was conducted. In this experiment 24 participants completed our computer game and filled out an existing personality questionnaire, the NEO-PIR [7]. Multiple Regression Analyses was used to test the correlations between the test items and the NEO-PI-R scores. Five of the items had a positive correlation with the NEO-PI-R extraversion-score, indicating that testitems in a virtual environment can actually be used to measure extraversion. We conclude that it is possible to measure personality traits, and consequently a valid psycholo...