The observation of luminescence from the Pr3+1S0 state in LaB3O6 under vacuum ultraviolet excitation is reported and discussed. The photon cascade emission consists of the S-1(0) --> I-1(6) and D-1(2) --> H-3(4) transitions. The efficient multiphonon P-3(0) --> 1D(2) nonradiative relaxation is responsible for the absence of P-3(0) luminescence in LaB3O6.