Tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) fruit, at five stages of development, have been analyzed for their carotenoid and chlorophyll (Chl) contents, in vitro activities of phytoene synthase, phytoene desaturase, and lycopene cyclase, as well as expression of the phytoene synthase (Psy) and phytoene desaturase (Pds) genes. During ripening, the total carotenoids increased with a concomitant decrease in Chl. Although the highest carotenoid content (consisting mainly of lycopene and [beta]-carotene) was found in ripe fruit, the greatest carotenogenic enzymic activities were found in green fruit. Phytoene synthase was located in the plastid stroma, whereas the metabolism of phytoene was associated with plastid membranes during all stages of fruit development. The in vitro products of phytoene desaturation altered from being predominantly phytofluence and [zeta]-carotene in chloroplasts to becoming mainly lycopene in chromoplasts. The expression of Psy was detected in breaker and ripe fruit, as well as flowers, but was not detectable by northern blot analysis in leaves or green fruits. The Pds gene transcript was barely detectable in green fruit and leaves but was expressed in flowers and breaker fruit. These results suggest that transcription of Psy and Pds is regulated developmentally, with expression being considerably elevated in chromoplast-containing tissues. Antiserum to the Synechococcus phytoene synthase cross-reacted with phytoene synthase of green fruit only on western blots and not with the enzyme from ripe fruit. In contrast, a monoclonal antibody to the Psy gene product only cross-reacted with phytoene synthase from ripe fruit. The enzymes from green and ripe fruit had different molecular masses of 42 and 38 kD, respectively. The absence of detectable Psy and Pds mRNA in green tissues using northern blot analyses, despite high levels of phytoene synthase and desaturase activity, lends support to the hypothesis of divergent genes encoding these enzymes.