Tomato fruit ripening is characterized by a decrease in chlorophyll content and an increase in lycopene synthesis. 5-Aminolevulinic acid (ALA) dehydratase (ALAD) is the fruit committed enzyme in the chlorophyll and heme biosynthetic pathways, and it catalyzes the dimerization of two ALA molecules into porphobilinogen We have focused our attention on the potential pivotal role of ALAD in the developmental regulation of chlorophyll biosynthesis during tomato fruit growth, development, and ripening. We have standardized an assay procedure for measuring the enzymatic activity of ALAD in tomato fruit tissues. The activity of ALAD was assayed from ten days past anthesis to day 60, when fruits where void of chlorophyll. We observed a several-fold decline in ALAD activity to residual levels during fruit ontogeny. Our data also show greater ALAD activity in chlorophyllous organs (leaves, stems, immature fruits) than in nonchlorophyllous organs (roots, ripe fruits), where heme production is predominant.