A theoretical model of single-domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two-domain configuration takes place at the SD threshold d0. This two-domain configuration is shown to be more applicable to fine-grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superamagnetic threshold size ds by N¿els relaxation equation and calculating the SD threshold d0 at which SD to two-domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds?0.05 μm, and d0?0.076 μm. For cubic magnetite particles just above d0 a two-domain configuration is predicted in which a 180¿ domain wall occupies ~60% of the particle volume. No SD range exists for cubic magnetites at T>450¿K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo-single-domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length: width ratio of 5:1, SD limits of ds?0.05 μm and d0?1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3-xTixO4) increase with Ti content. For cubic titanomagnetities of x=0.6, ds?0.08 μm, and d0?0.3 μm. Comparison of the calculated SD range with the available high-resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks.