The intensity and stability of thermoremanent magnetization (TRM) acquired by fine particles (<1 μm) of synthetic monoclinic pyrrhotite were determined. A specific TRM (magnetization per unit inducing field) of 32 with a median destructive field of 36 kA m-1 was observed. The median destructive field of saturation isothermal remanence is 42 kA m-1, and the coercive force is 62 kA m-1. The intensity of TRM has been compared to that predicted by the classical monodomain model adapted to the magnetic symmetry of pyrrhotite. If the optically determined particle size is incorporated into the model, the agreement between model and observation is poor (a factor of 100 difference); if a grain size (~0.05 μm) inferred from the measurement of coercive force is adopted, model and observation are in agreement within experimental error. ¿ American Geophysical Union 1996