This paper presents a model describing strain localization in single crystals. Constitutive relations for slip systems axe proposed within the framework of thermodynamics. A generalized Schmid law is formulated in terms of the Eshelby stress tensor. The strain localization is found to be possible when the hardening rate for active slip systems has fallen to a certain critical positive value hcr. The latter is determined for double active slip systems. A comparison with corresponding results available in the literature is provided.