In the title compound, C21H17N2P, the dihedral angles between the 1,5-naphthyridine ring system (r.m.s. deviation = 0.005 Å) and the phenyl rings are 89.18 (8) and 77.39 (8)°. The phenyl rings are almost perpendicular, making a dihedral angle of 88.12 (8)°. The only possible inter-molecular inter-action is a very weak aromatic π-π stacking inter-action [centroid-centroid separation = 3.898 (2) Å].