An ac bridge method has been used to study the effect of uniaxial compression on the initial magnetic susceptibility of rocks and separated titanomagnetites. The samples used were basalts containing titanomagnetite with varying grain size and morphology and dispersions of magnetite and titanomagnetite powders in epoxy. The results indicate that (1) the effect of pressure on susceptibility decreases with decreasing grain size, not in a continuous manner but rather depending upon whether the dominant magnetic grain size is multidomain, pseudo single-domain or single-domain, (2) the pressure response increases with the composition parameter x of the titanomagnetite in the solid solution series xFe2TiO4 (1-x)Fe2O4, and (3) the morphology of the grains influences the pressure response. The results are interpreted in terms of the behavior of the multidomain, pseudo single-domain and single-domain material. Changes in thermoremanent magnetization and the acquisition of pressure remanent magnetization under uniaxial compression, which were observed in conjunction with the susceptibility studies, suggest that rocks containing coarse grain titanomagnetites as the dominant magnetic mineral phase are the most efficient stress transducres. Hence a definitive seismomagnetic experiment would be easiest in a seismically active region in which such rocks are prominent.