A synthetic thermoremanent magnetization (TRM) induction curve is produced from domain observations of an ensemble of pseudosingle domain (PSD) titanomagnetite particles in oceanic basalts. The observations were made after the samples were given a TRM in fields of 0.1, 2, 4, 6, and 8mT. An observation was also made of the domain state for saturation isothermal remanent magnetization (IRMs). The number and maximum size of grains which remain in a metastable single domain (SD) state increases with increasing field. This in turn gives rise to a field dependence of the total moment carried by such particles in the ensemble. By observing the number of metastable SD grains and estimating their volume one can construct a TRM induction curve. This curve approximates the known TRM induction curve for fine particles, demonstrating that the magnitude of TRM in fine particles of titanomagnetites is controlled by nucleation. This nucleation model also explains why weak field TRM is more stable against alternating field (AF) demagnetization than IRMs. ¿ American Geophysical Union 1988