In red soils from southern Brazil magnetite was reported to be pedogenically unstable, weathering to maghemite. However, in similar soils from northeastern Argentina magnetite was found in all size fractions. This finding motivates the mineralogical study of an Ultisol at different depths in order to understand the influence of anthropic and natural factors in the weathering of the magnetic minerals of these subtropical soils. The sand fraction of the B_t22 horizon (105-155 cm depth) of a clayey red Ultisol from the subtropical forest of Misiones, Argentina, was studied by X-ray diffraction, saturation magnetization σ_s, optical microscopy and Mössbauer spectroscopy. Saturation magnetization for the whole sand fraction (wsf), the non-magnetic sand fraction (nmsf) and the magnetic sand fraction (msf) are 10.79, 1.50 and 16.92 JT^-1 kg^-1, respectively. Mainly quartz, ilmenite, Al-substituted hematite, goethite, maghemite and magnetite are found. Magnetite-maghemite contents are high, and magnetite is predominant in the msf. Results are compared with those from the upper B₁ horizon (10.35 cm depth) of the same soil in which a lower σ_s(wsf) value, and higher values of σ_s(msf) and of (σ_s(msf)-σ_s(wsf)) were measured. These results confirm the stability of magnetite in this soil contrasting with other results on soils from neighbouring areas.