AB block copolymers with an isotropic and a nematic block are synthesized by living anionic copolymerization of isotropic and mesogenic methacrylates. The complete phase diagrams of the corresponding homopolymers with the nematic solvent have been established. In dilute solution (0.2 to 5% by weight) in a low-molar-mass nematic solvent, the copolymers exhibit two different types of structure formation. Polymers with high isotropic block fraction form irreversible networks as a result of phase separation followed by vitrification of the polymer-rich phase. Polymers with high nematic block fraction on the other hand form thermoreversible threadlike structures perfectly ordered parallel to the director of the nematic solvent. These structures cannot be explained by a macroscopic phase separation process but more likely by a formation of micellar-like aggregates.