The biophysical mechanism of vertebrate magnetic sensory perception has not been completely resolved. We here provide evidence for the use of a magnetic material (probably magnetite) by a vertebrate to detect the earth's magnetic field. The role of magnetite in bobolink (Dolichonyx oryzivorus) orientation was assessed by magnetizing the birds with a magnetic pulse in one of three orientations. Bobolinks magnetized with different polarities were significantly oriented in directions different from one another and from their controls. Treatment with a second pulse having the opposite polarity to the first resulted in random orientation for each group. These results indicate an effect specific to a particle-based magnetoreceptor. The use of magnetite particles for magnetoreception is not in conflict with other reports on the use of photopigments for this purpose. The two mechanisms could be used in a complementary manner for detecting the same or different aspects of the magnetic field.