Guided waves in a general class of waveguides with biisotropic filling are studied. The biisotropic medium is the most general isotropic linear medium, that is, nonreciprocal chiral medium. Field equations are derived first for arbitrary waveguide cross sections and then applied for a special circular cross section. The following special boundary impedance conditions have been covered: ideally conducting surface, isotropic, and anisotropic impedance surface and open surface. Some general properties of the eigenwaves are established, and the influence of nonreciprocity together with chirality and losses is discussed. Calculated curves are shown for biisotropic circular waveguides with ideally conducting and isotropic impedance surfaces.