The cellulolytic activity of sodium chloride extracts of four portions of tomato fruits--seeds and pulp, placenta, inner pericarp, and outer pericarp--was determined over a maturity range extending from green fruits less than 1 inch in diameter to ripe fruits (8 days after turning color at the blossom end). Generally the green fruits were low in activity and the fruits showing red color were high. The outer pericarp had a high activity at the earliest green stage with the activity decreasing to the turning stage. The seeds and pulp had a high activity just prior to and during the mature-green stage coincident with the breakdown of the pulp around the seeds. The placenta had a much higher activity than the other portions after the mature green stage. The activity of each portion was constant during the ripening period from 2 to 8 days after the turning stage.