Rock magnetic studies were carried out in order to identify the remanence carrying minerals and the domain state of magnetic grains in Lower/Middle Pleistocene marine sediments from the Wanganui Basin, southwestern North Island, New Zealand. Isothermal remanent magnetization (IRM) acquisition and stepwise thermal demagnetization of a three-axis composite IRM suggest that titanomagnetite (or titanomaghaemite) is the dominant remanence carrier in these sediments. Other rock magnetic data suggest that these grains occur in a range of sizes in the superparamagnetic to pseudo-single domain range, which helps account for the wide range of observed palaeomagnetic behavior. Isolated occurrences of ferrimagnetic iron sulphides cannot be ruled out. ¿ American Geophysical Union 1993