A neutron burst from an untamped cylindrical Oy assembly occurred at the Pajarito Remote Control Laboratory at 1800 hours on 18 April 1952. The circumstances causing the burst and a description of the assembly involved are given There was no personnel hazard. Normal operations could have been resumed on other assemblies withing two or three hours after the burst. No evidence of damage to the oy was observed. It was determined that the burst resulted in 1.5 x 10(16) fissions in the oy. Also reported are some results of the computations and post-burst tests and a discussion of probable time behavior of power level during the burst.