Samples from DSDP Leg 37 holes 332B, 334, and 335 have been subjected to magnetic and microstructural investigation. A strong, stable remanent magnetization is observed, with the high Koenigsberger ratios that are characteristic of oceanic basalts. The remanence is generally carried by dendritic s ingle-domain and (or) pseudo-single-domain oxide grains, which are homogeneous. However, thermal demagnetization in air leads to the formation of magnetite–ilmenile intergrowths not detectable by optical microscopy but clearly visible in the electron microscope. Such ultra-fine phase splitting has often been invoked as an explanation of magnetic changes observed during laboratory heating.