The causes of the black discoloration of three species of shrimp commonly caught off the coast of the State of São Paulo, Brazil, are studied and measures for its prevention are advocated. Landings of shrimp in Santos in 1964 equaled 13% of the total landing, by weight, while economically it is the principal fishery. I - Blackening was effectively prevented by dipping the shrimp in cooled chemical solution for 30 minutes and then storing them at low temperatures. L-ascorbic acid gave the best results, delaying discoloration up to 9 days at 0º to -2ºC and for a longer period at colder temperatures. The effect of Sodium-thiosulfate and EDTA at different temperatures is also discussed. II - Tyrosinase was isolated and assayed by measuring the formation of Dopa-chrome from DL-Dopa at pH 6.8 an 37ºC. Tyrosinase was obtained from the shrimp liver and was purified by approximately 25 times its activity from the initial extract by absorption chromatography through Celite 545, as judged by the rate of dopachrome formation.