The investigation of the causes of a fish kill in waters containing ferro‐ and ferricyanide at concentrations far under those generally accepted as non‐lethal have shown these low concentrations to be lethal due to photo‐decomposition and release of the cyanide ion. Experimental data place the toxic level of these compounds, under similar conditions of light intensity, at a concentration between 1 and 2 p.p.m. This level is assumed to vary slightly under natural conditions with latitude, period of the year, temperature, water turbulence, and interferences. Rapid development of toxicity has been demonstrated at a concentration of 2 p.p.m. of either potassium ferro‐ or ferricyanide.