The reorientation of an initially well aligned nematic rodlike solution of the rodlike poly(1,4-phenylene-2,6-benzobisthiazole), PBT, in an external magnetic field in the sample plane is studied. For angle β between the magnetic fieldHand the initial directornless than 45 degrees, the reorientation occurs by a homogeneous twist distortion of the director field, permitting assessment of the Frank elastic constant KTand the Leslie viscosity ηT.For larger β, the reorientation is nonhomogeneous, with a component of the distortion out of the sample plane, resulting in a phase-grating, made manifest by its focusing effects for transmitted light.