The theory of poromechanics, most often used for macroscopically isotropic granular material, is applied to unidirectional fiber assemblies. When the deformation of the constituting fibers of those assemblies is taken into account during undrained forced compression, the Biot coefficients are expected to be lower than one. In the case of transversely isotropic materials, two independent Biot coefficients exist, one according to the fiber length direction and another relative to the plane perpendicular to that direction. The present paper aims at predicting the Biot coefficients for perfectly straight unidirectional (UD) fiber assemblies using micromechanical methods: dilute, Mori–Tanaka and Ponte Castañeda–Willis estimates. An experimental procedure, based on both uniaxial and triaxial material testing machines, is also detailed and applied to rubber fiber assemblies. The theoretical and experimental Biot coefficient results are compared and show a good agreement.