Poly(benzobisthiazole)s containing an o-tetramethyl substituted biphenyl moiety were synthesized based on 2,S-diamino-l,4-benzenedithiol dihydrochloride and 2,2',6,6'-tetramethylbiphenyl-4,4'-dicarbonyl chloride. Homopolymerization of the two monomers and copolycondensation of the two monomers with terephthaloyl chloride were carried out in poly(phosphoric acid) (PPA). The interfacial polymerization technique was used to enhance the formation of a homogeneous liquid crystalline polymer solution suitable for fiber spinning. Copolymers of tetramethylbiphenyl PBZT/PBZT (25/75) were successfully spun into fibers from liquid crystalline solutions in PPA. Fibers were thermally cross-linked between 500 and 550 degrees C. The thermal cross-linking reaction was studied by thermogravimeteric analysis and by C-13 solid-state NMR spectroscopy. Fiber morphology and properties are also reported and discussed.