The dynamic stability of a poroelastic column subjected to a longitudinal periodic force is investigated. The column material is assumed to be transversely isotropic with respect to the column axis, and the pore fluid flow is possible in the axial direction only. The motion of the column is governed by two coupled equations, for which the stability boundaries are determined analytically by using the multiple-scales method. It is shown that due to the fluid diffusion the stability regions are expanded, relative to the elastic (drained) case. The critical (minimum) loading amplitude, for which instability occurs, is also given.