To develop and validate an ultra-short measure to assess the Big Five in social network designs, the unipolar items of the Ten-Item Personality Inventory were adapted to create a bipolar single-item scale (TIPI-r), including a new Openness item. Reliability was examined in terms of the internal consistency and test–retest stability of self-ratings and peer-rating composites (trait reputations). Validity was examined by means of convergence between TIPI-r and Big Five Inventory (BFI) scores, self-peer agreement and projection (intra- individual correlation between self- and peer-ratings). The psychometric quality of the TIPI-r differed somewhat between scales and the different reliability and validity criteria. The high reliability of the peer-rating composites motivates to use the TIPI-r in future studies employing social network designs. Copyright © 2007 John Wiley & Sons, Ltd.