The zirconium-alizarin sulfonate indicator of Lamar has been applied to the spectrophotometric determination of fluoride in sea water and has been shown to be suitable. Calibration curves have been prepared for different chlorinities. The variation of apparent fluoride content with chlorinity is less than that reported by Thompson and Taylor with their ;eagent. Borate, iron and phosphate have been shown not to interfere at the concentrations found in sea water, while a method has been given to take care of varying alkalinities. The effects of time, temperature and light upon the color intensity have been discussed. Increased accuracy is attained through the measurement of the color intensity spectrophotometrically.