A variational criterion for the study of micropolar continuous media in which dissipative phenomena occur is proposed. This criterion is an extension of Lebon-Lambermont's, which is recovered when the spin becomes zero (structureless media). The Euler-Lagrange equations obtained from the variational principle are the balance equations for a micropolar medium with microisotropy. As application of the criterion, one treats numerically the Poiseuille flow of a micropolar medium. © 1981.