The changes in ethylene production rates and development of 1-aminocyclopropane-l-carboxylic acid (ACC) synthase and polygalacturonase (PG) activities were studied during the maturation and ripening of tomato fruit (Lycopersicon esculentum Mill, cv. ‘Castlemart’). There was a linear relationship between internal ethylene concentration and ethylene production rate; both increased exponentially as tomato fruit reached more advanced maturity and ripening. Thus, both of them correlate with the maturity and the ripening stages of tomatoes. A small increase in ACC synthase activity was observed at the early mature green stages which was followed by a marked increase at the breaker stage. ACC level followed the same pattern as ACC synthase activity. PG activity was undetectable or low throughout the mature green stages, but increased significantly after reaching the breaker stage. These data indicate that the onset of the development of ACC synthase activity precedes that of PG activity.