Carotenoids are plastidic isoprenoid pigments of great biological and biotechnological interest. The precursors for carotenoid production are synthesized through the recently elucidated methylerythritol phosphate (MEP) pathway. Here we have identified a tomato ( Lycopersicon esculentum Mill.) cDNA sequence encoding a full-length protein with homology to the MEP pathway enzyme hydroxymethylbutenyl 4-diphosphate synthase (HDS, also called GCPE). Comparison with other plant and bacterial HDS sequences showed that the plant enzymes contain a plastid-targeting N-terminal sequence and two highly conserved plant-specific domains in the mature protein with no homology to any other sequence in the databases. The ubiquitous distribution of HDS-encoding expressed sequence tags (ESTs) in the tomato collections suggests that the corresponding gene is likely expressed throughout the plant. The role of HDS in controlling the supply of precursors for carotenoid biosynthesis was estimated from the bioinformatic and molecular analysis of transcript abundance in different stages of fruit development. No significant changes in HDS gene expression were deduced from the statistical analysis of EST distribution during fruit ripening, when an active MEP pathway is required to support a massive accumulation of carotenoids. RNA blot experiments confirmed that similar transcript levels were present in both the wild-type and carotenoid-depleted yellow ripe ( r) mutant fruit independent of the stage of development and the carotenoid composition of the fruit. Together, our results are consistent with a non-limiting role for HDS in carotenoid biosynthesis during tomato fruit ripening.