A rapid, inexpensive enzymatic method is proposed for screening water quality in the laboratory, on board ship or on-site. The method uses the enzyme acetylcholinesterase extracted with Triton X-100 from the supernatant of a homogenate of the cladoceran, Moina macrocapa. Modification of the specific activity of the enzyme compared to controls is measured during incubation of the enzyme extract with water from various sources. This study demonstrated differential response to enzyme activity in water from a large reservoir (Ignacio Ramírez) compared with effluent from an oil refinery (18 de Marzo). In vitro AChE activity is suggested as a rapid test to identify water samples that are toxic or of poor quality and that should be subject to further chemical analysis and toxicity testing.