Tyrosine/dopa decarboxylase (TYDC) catalyzes the formation of tyramine and dopamine and represents the first steps in the biosynthesis of the large and diverse group of tetrahydroisoquinoline alkaloids. Opium poppy accumulates morphine in aerial organs and roots, whereas sanguinarine, which is derived from a distinct branch pathway, accumulates only in roots. Expression of the TYDC gene family in opium poppy was investigated in relation to the organ-specific biosynthesis of these different types of alkaloids. Members of the TYDC gene family are classified into two groups (represented by TYDC1 and TYDC2) and are differentially expressed. In the mature plant, TYDC2-like transcripts are predominant in stems and are also present in roots, whereas TYDC1-like transcripts are abundant only in roots. In situ hybridization analysis revealed that the expression of TYDC genes is developmentally regulated. TYDC transcripts are associated with vascular tissue in mature roots and stems but are also expressed in cortical tissues at earlier stages of development. Expression of TYDC genes is restricted to metaphloem and to protoxylem in the vascular bundles of mature aerial organs. Localization of TYDC transcripts in the phloem is consistent with the expected developmental origin of laticifers, which are specialized internal secretory cells that accompany vascular tissues in all organs of select species and that contain the alkaloid-rich latex in aerial organs. The differential expression of TYDC genes and the organ-dependent accumulation of different alkaloids suggest a coordinated regulation of specific alkaloid biosynthetic genes that are ultimately controlled by specific developmental programs.