Magnetic field effects on the excitation spectra of the 1s triplet exciton luminescence in monoclinic -ZnP 2 are observed at 2 K. Under a magnetic field of 1 T, the luminescence splits into two components with a separation of about 0.2 meV. The excitation spectrum for each of Zeeman-split luminescence components is measured. The spectrum for one component is definitely different from that for the other. Excitation into some of the excitation peaks produces only one component of the luminescence. It is concluded that the triplet excitons relax through a selective path. Relaxation processes through the multiple phonon scatterings are discussed, and possible processes through optical dipole transitions in the triplet exciton system of -ZnP 2 are also proposed.