The low ac- and dc-field magnetic susceptibilities of hexagonal YbMn O3 single crystal have been measured along the ab plane and along the c axis (ab, c). The antiferromagnetic Ńel temperature TN is well signaled by a cusp in the temperature dependence of c but no features are visible in ab. A remarkable anisotropy of paramagnetic susceptibility and the extrapolated Curie-Weiss temperature are observed and related to the quasibidimensional magnetic structure of YbMn O3. Below TN magnetic irreversibility sets, reflecting the presence of a weak ferromagnetic response. Possible scenarios for the origin of the weak ferromagnetism are discussed. © 2008 American Institute of Physics.