Synthetically grown and natural multidomain magnetite crystals (d≥30 &mgr;m) have strongly temperature-dependent magnetic domain structures. The unblocking of domain walls at moderate temperatures (T<120 ¿C) makes these magnetic grains unsuitable as paleomagnetic recorders. Approximately 15% of the smaller hydrothermally grown magnetite crystals (d≤10 &mgr;m) show stable domain patterns when heated to 350 ¿C. These magnetites with temperature-independent domain structures can possibly account for paleomagnetic remanences with high blocking temperatures. ¿ American Geophysical Union 1988