The Zeeman splitting of the phosphorescence (T1→ S0) was investigated in a mixed crystal of perdeuterated Naphthalene (N-d8) and Naphthalene (N-h8). The N-h8 concentration varied between 0.2 and 5%, the temperature was 4.2 °K and the magnetic field strength was 50 kG. The intensity of the Zeeman components is strongly dependent on the direction of the field with respect to the molecular axes. In addition to this anisotropy we observe a strong anisotropy of the lifetime of the triplet state. Both these anisotropics are due to the zero field splitting of the triplet state T1. Their analysis yields the following constants: (1) the relative radiative transition probabilities kxr, kyr and kzr from the three zero field components Tx, Ty and Tz to the singlet ground state S0 and (2) the total lifetimes tx , ty and tz of each zero field component. No assumptions concerning selection rules are made in the analysis. The results for the values of the kur (u-x, y, z) depend on the vibronic state of S0: if totally symmetric states are excited, 97% of the phosphorescence originates from the zero field component Tx . If the Δṽ= 1629 cm-1 B3g-vibration is excited the phosphorescence originates mainly from Ty and Tz . The total lifetimes of Tx , Ty and Tz are the same as measured previously in low fields by ESR.