[en] The development of a large water Cherenkov counter for use in triggering over a large side aperture of the Brookhaven National Laboratory Multi-Particle Spectrometer is described. Design considerations, prototype-testing, and testing and utilization of the full-scale counter are discussed. The water radiator occupied a volume of 7.6x102x229 cm³; the large walls of the radiator box were constructed of PVC foam with aluminum skins, and were supported only at the periphery. Light was collected vertically along the 102 cm dimension, with one 2 ins photomultiplier (RCA 4507) employed for each of the eight cells. The major inside reflectors were aluminized mylar sealed within a glass envelope for reasons of durability and also of total internal reflection within the glass. With highly relativistic particles, efficiencies of 99.2% or greater were obtained at midplane from all cells; efficiencies decreased one percent or less near the bottom of the cells. Light produced by knock-on electrons from low-momentum protons was found to increase slowly with radiator thickness. This light was discriminated against with high efficiency. The Cherenkov counter was used as a major trigger element to distinguish side-pion from proton-recoil events in a recent MPS experimental run at the Brookhaven Alternating Gradient Synchrotron. (Auth.)