Optically detected multipulse nuclear quadrupole resonance is used to study trivalent praseodymium in lanthanum trifluoride in zero and weak static magnetic fields. A spin-locking spin-echo rf pulse sequence consisting of a π/2 preparation pulse, followed by repeated sequence of space t, a π/2 pulse phase shifted by 90° from the preparation pulse, and a space t. A Raman heterodyne scheme was used for optical detection. The coherence was observed to decay exponentially at a rate of 51 μsec (6.2 kHz) in zero static magnetic field. For static magnetic fields of 80 G applied along the c3 axis the decay was described by time constants of 76 μsec (4.2 kHz) and 372 μsec (855 Hz). This represents a lengthening of the coherence time from a previously reported (two-pulse spin echo) 16.9 μsec (18.8 kHz) at zero field and a measured 37.5 μsec (8.5 kHz) at 80 G.