Sodium pentachlorophenate can be quantitatively estimated in natural waters by observing the mortality rate of the common guppy, Lebistes reticulatus. The test requires evaluation of mortality rate curves for specific dosages covering the range of conditions to be encountered in the field. Quantitative field determinations are obtained from a comparison of the unknown with the standard mortality curves. Use of this method gives reliable results necessary for effective mollusciciding operations. Materials required are those normally found in the field.