To establish links among domain images, volume changes in magnetization, and mechanisms for pseudosingle-domain (PSD) behavior, we have studied single platelets of natural hematite (Elba, Italy) in order to compare Bitter patterns resulting from high-field treatments to the hysteresis behavior of the same individual particles. There is excellent agreement between the fields that cause large changes of a particle's Bitter pattern and the fields that cause large changes of a particle's magnetic moment. When in the saturation remanent state, most particles exhibit patterns that suggest a state of near saturation. Such patterns change very little until the application of a critical back field triggers the appearance of a major Bitter line that nearly bisects the grain. Likewise, in the majority of particles the measured ratio of saturation remanence to saturation moment is large (≂0.7), and there is little change in saturation remanence until a critical back field equal to the coercive force (Hc) reverses the moment through a large Barkhausen jump. In most particles whose patterns have been compared to their measured behavior, this large jump on the hysteresis loop occurs in the same critical back field that causes the sudden appearance of a major Bitter line. Consequently, we interpret hysteresis to be governed by the nucleation of walls in the majority of hematite particles studied here. In such particles, Hc increases as effective grain size decreases, according to Hc∝deff-0.64, where deff∝Ms1/3 and Ms equals the particle's saturation moment. This study leads to four conclusions: (1) Bitter patterns do represent volume domains and the magnetization processes that they imply, (2) nucleation is the dominant hysteresis mechanism in PSD particles of this natural hematite, (3) the grain size dependence of nucleation field Hn in hematite is given by Hn=Hc∝deff-0.64, and (4) nucleation is a viable mechanism for explaining PSD behavior in other natural magnetic materials, such as magnetite, titanomagnetite, and pyrrhotite. ¿ American Geophysical Union 1995