Analysis of exoglucanase activity accumulated by sec mutants from Saccharomyces cerevisiae revealed the presence of two ionic forms of the major exoglucanase (exo II) secreted into the culture medium. From the accumulation pattern of representative sec mutants and the carbohydrate composition it appears that the less acidic form is converted into the more acidic one by addition of one phosphate to one of the oligosaccharide cores as the enzyme progresses through the secretory pathway. Exoglucanase I, the heavier isoenzyme, was not accumulated by the mutants. Accordingly, it should arise from exoglucanase II after the execution point of sec1 mutation.