The genomic DNA sequence of Synechocystis was analysed for putative ζ-carotene desaturase genes. Two promising candidates slr0940 and slr0033 were found with similarities to the structurally different ζ-carotene desaturase genes from higher plants and Anabaena, respectively. Only the expression product of the analogue to the plant gene, slr0940, was able to mediate the 2-step desaturation of ζ-carotene via neurosporene to lycopene after complementation of this pathway in Escherichia coli. When enzyme reactions were carried out with this protein, activity was obtained with either ζ-carotene or neuroporene as substrates. The in vitro reaction was inhibited by the pyrimidine derivative J852 which is effective as experimental herbicide in plants. The occurrence of two different types of ζ-carotene desaturases among cyanobacteria and the phylogenetic consequences on chloroplast evolution are discussed.