Magnetic experiments conducted on leg 37 basement rock include granulometric techniques (study of low- and high-field hysteresis and temperature dependence of initial susceptibility and of magnetization) and the Lowrie-Fuller test. On the basis of these experiments the following inferences were made concerning the magnetic domain structure: The effective particle size of only one sample, basalt from hole 332B, is in superparamagnetic size range. The single sample of periodotite (from hole 334) provided the only evidence for the presence of multidomain particles. The behavior of the remaining (44) samples, all basalts, can be interpreted as being due to single domain particles which in some cases appear to be moderately to strongly cation deficient. Magnetite or titanomagnetite was found to be the only significant magnetic carrier present.