Preliminary data are presented on the physical properties and the mineralogy of the material in denticle caps of chitons. The X-ray diffraction patterns of the denticle material indicate the presence of magnetite, or possibly maghemite. This is the first indication that magnetite or possibly maghemite is precipitated by a biologic agent in sea water. Hence, the origin of magnetite reported in recent and fossil marine deposits will have to be re-examined. The hardness of the denticle material (Ca. 6 on the Mohs scale) explains why chitons are effective erosional agents of rock surfaces, particularly of limestone. From the biologic point of view the possibility is considered that the magnetic properties of the denticle caps may serve as a guidance system for the so-called homing instinct of chitons.