The development of a sensitive screening procedure for mutants of Tetrahymena thermophila blocked in secretion of lysosomal enzymes is described. By means of this procedure a mutant blocked in secretion of lysosomal enzymes has been isolated. This sec- mutant, MS-1, is constitutively blocked in release of at least six lysosomal enzymes, under both nutrient and non-nutrient conditions. MS-1 possesses, bound within the cell, the same amount of active lysosomal enzymes as the wild type. During starvation in media of low ionic strength MS-1 develops a highly vacuolated phenotype. This phenotype is caused by the sec- allele. It is reversed to a normal cell shape when the mutant is transferred to isotonic medium. The sec- mutant MS-1 contains mucocysts and is capable of inducing exocytosis of these secretory organelles, suggesting that Tetrahymena possesses at least two independent protein-secreting organelles.