A wide variety of macrocyclic tetrapyrrole compounds are synthesized by cells for use in diverse cellular processes. As shown in Fig. 1, this pathway includes the synthesis of closed tetrapyrrole compounds that contain metals, such as cobalt-containing cobalamin (vitamin B12), nickel-containing methanogenesis coenzyme F430, copper-containing pigment turacin, iron-containing heme and siroheme, and magnesium-containing chlorophylls and bacteriochlorophylls. Also synthesized by this pathway are a variety of open macrocyclic compounds such as the family of bilins that are used as a source of light-harvesting pigments in cyanobacteria, red algae, and cryptophytes as well as the chromophore that is covalently attached to phytochrome apoproteins in plants. Several comprehensive reviews which focus on the biochemistry and enzymology of the various branches of the tetrapyrrole biosynthetic pathway have recently been published (6, 22, 51). Genetic analysis of the tetrapyrrole biosynthesis pathway is an active area of study in a number of laboratories. Over the past decade, there has been significant progress in obtaining molecular genetic information on loci involved in the early part of the pathway leading from 5-aminolevulinic acid to heme (33). Loci involved in vitamin B12 synthesis have also been characterized at the molecular genetic level (33). Molecular genetic information on loci involved in the Mg-tetrapyrrole branch of the biosynthesis pathway has, however, only recently been obtained. It is this latter area that this minireview will cover, particularly the recent use of bacterial sequence information to aid in defining the roles and evolutionary relationships of similar loci from plant and algal systems.