Ferruginous minerals in polymetallic nodules from the East Pacific basin were studied by means of transmission electron microscope (TEM). Nine species of ferruginous minerals were identified in the nodules, namely ferocyhyte, akagancite, goethite, ferrihydrite, hematite, magnetite, maghemite, chromite and the hisingerite-like mineral. Both feroxyhyte and vernadite assume curled fibrous aggregates under TEM. It is considered that part of Fe3 converted nonbiologically from Fe2 in the ooze is trapped and then precipitates as feroxyhyte with vernadite during the deposition of Mn4. Goethite is too small to be recognized even under TEM. It was for the first time that ferrihydrite was discovered in oceanic nodules. The fact that the electron diffraction pattern of ferrihydrite contains the electron diffraction of goethite suggests the reciprocity between them. Hisingerite-like mineral which mainly consists of Si and Fe has been discovered both in polymetallic nodules and in sediments. This mineral was previously considered to be a mixture of ferruginous minerals and silicates. As samples of this mineral from various places are similar to hisingerite in structure, TEM morphological features and chemical composition, the authors believe that it is an independent mineral instead of a mixture, and hence temporarily name it hisingerite-like mineral.