An analysis is presented of perfectly-conducting cylindrical structures completely filled with cold, anisotropic plasmas. Major, but not exclusive, consideration is directed to the circular waveguide containing a lossless, longitudinally magnetized plasma. The modes are generally a mixture of TE and TM fields with the result that the conventional orthogonality relations must be replaced by appropriate generalizations. The orthogonality relations are used to determine the fields excited in a longitudinally magnetized plasmaguide*by a coaxial current loop. At cut-off, resonance, and at limiting values of the system's parameters, the modes reduce to simple TE or TM waves. The field configuration and dispersion are examined at these limiting conditions as well as at intermediate values of the parameters. Numerical results are provided for the cut-off frequencies and dispersion relation. *Plasmaguide is the generic name given to any metallic waveguide containing an anisotropic plasma. In this manuscript it will refer to the longitudinally-magnetized, plasma-filled waveguide.