<jats:title>ABSTRACT</jats:title> <jats:p> <jats:italic>Cyanidioschyzon merolae</jats:italic> is considered to be one of the most primitive of eukaryotic photosynthetic organisms. To obtain insights into the origin and evolution of the pathway of carotenoid biosynthesis in eukaryotic plants, the carotenoid content of <jats:italic>C. merolae</jats:italic> was ascertained, genes encoding enzymes of carotenoid biosynthesis in this unicellular red alga were identified, and the activities of two candidate pathway enzymes of particular interest, lycopene cyclase and β-carotene hydroxylase, were examined. <jats:italic>C. merolae</jats:italic> contains perhaps the simplest assortment of chlorophylls and carotenoids found in any eukaryotic photosynthetic organism: chlorophyll <jats:italic>a</jats:italic> , β-carotene, and zeaxanthin. Carotenoids with ε-rings (e.g., lutein), found in many other red algae and in green algae and land plants, were not detected, and the lycopene cyclase of <jats:italic>C. merolae</jats:italic> quite specifically produced only β-ringed carotenoids when provided with lycopene as the substrate in <jats:italic>Escherichia coli</jats:italic> . Lycopene β-ring cyclases from several bacteria, cyanobacteria, and land plants also proved to be high-fidelity enzymes, whereas the structurally related ε-ring cyclases from several plant species were found to be less specific, yielding products with β-rings as well as ε-rings. <jats:italic>C. merolae</jats:italic> lacks orthologs of genes that encode the two types of β-carotene hydroxylase found in land plants, one a nonheme diiron oxygenase and the other a cytochrome P450. A <jats:italic>C. merolae</jats:italic> chloroplast gene specifies a polypeptide similar to members of a third class of β-carotene hydroxylases, common in cyanobacteria, but this gene did not produce an active enzyme when expressed in <jats:italic>E. coli</jats:italic> . The identity of the <jats:italic>C. merolae</jats:italic> β-carotene hydroxylase therefore remains uncertain.