The present invention provides binding moieties for fibrin which have a variety of uses wherever detecting, isolating or localizing fibrin, and particularly fibrin as opposed to fibrinogen, is advantageous. Particularly disclosed are synthetic, isolated polypeptides capable of binding fibrin and recognizing the form of polymerized fibrin found in thrombi. In addition, the polypeptides have a slow dissociation rate from fibrin, which improves their ability to form a contrast image at the site of a fibrin clot, making the disclosed binding moieties particularly useful as imaging agents for thrombi.