During the rapid mineralization in soil of sucralose (4-chloro-4-deoxy-alpha,D-galactopyranosyl-1,6-dichloro-1,6-dideoxy-beta,D-fructofuranoside), a metabolic product was formed that appears to be the corresponding unsaturated aldehyde. During the slow and incomplete mineralization of sucralose in lake water, which was not increased by the addition of nitrogen and phosphorus, the same compound was produced. That product was further metabolized by microorganisms in lake water and soil. Mineralization was also slow in sewage under aerobic conditions, but organic products were not detected. Little or no CO2 was formed from the disaccharide in flooded soil or anaerobic sewage. Bacteria in culture did not use sucralose as a carbon source but did convert it to the presumed unsaturated aldehyde, 1,6-dichloro-1,6-dideoxy-D-fructose and possibly the uronic acid of sucralose. Sucralose carbon was not incorporated into cells of two sucralose-metabolizing bacteria or the microbial biomass of sewage or fake water. The chlorinated disaccharide was slowly metabolized by a galactose oxidase preparation. It is concluded that the chlorinated sugar is acted on microbiologically by cometabolism.