We investigated magnetic properties of mafic rocks recovered from Ocean Drilling Program Hole 794D in the Yamato Basin of the Japan Sea. In contrast to other back-arc basins in the western Pacific, lineated magnetic anomaly patterns have not been recognized in the Yamato Basin. Our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks. The dominant magnetic mineral of Hole 794D is titanomaghemite as commonly observed for typical oceanic basalts, but the grain size is much larger (greater than several tens of microns). Remanence properties, i.e., low natural remanent magnetization intensities (≈1.5 A/m), low Koenigsberger ratios (≈1.0), high viscous remanent magnetization, and mixed magnetic polarities, indicate that the mafic rocks cannot be a source of lineated magnetic anomalies. While Curie temperature, saturation magnetization, and susceptibility values are not significantly different from those of typical oceanic basalts, magnetic properties related to stability are greatly reduced. The large grain size of the mafic rocks from Hole 794D plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts, and is caused by the formation of sill-sediment complexes beneath the Yamato Basin. Sill-sediment complexes, rather than extrusive basalts in typical oceanic crust, are responsible for the absence of lineated magnetic anomalies in the Yamato Basin. ¿ 1998 American Geophysical Union