External magnetic field effects on the intensity and decay of the fluorescence of pyrimidine-d(4) have been examined with excitation at the individual rotational lines of the 0-0 band belonging to the S-0 --> S-1 transition. The fluorescence is quenched by a magnetic field with a tendency that the efficiency of quenching becomes higher with increasing J’ of the excited level, except for the R(0) and P(2) line excitations. The fluorescence lifetime, which tends to increase with increasing J’ at zero field, is also affected by the magnetic field. A good correlation is found between the field dependence of the fluorescence intensity and the field dependence of the fluorescence lifetime; the lifetime becomes longer when the fluorescence quenching becomes larger or vise versa. Field-induced quantum beats are observed on some rotational line excitations, and the fluorescence intensity and lifetime largely change at the field strengths when these beats are induced, The magnetic field effects are interpreted in terms of the singlet-triplet mixed-state model.