We have sequenced a plastid gene cluster from the unicellular red alga Cyanidium caldarium which is located downstream from the psbA gene and contains, in the following order, genes for a beta-allophycocyanin-like protein (apcB'), a putative 9.5 kDa allophycocyanin linker protein (apcL9.5) and a putative 29 kDa phycocyanin linker protein (cpcL29). The apcB' and apcL9.5 genes are organized in the form of an operon. The cpcL29 gene is transcribed monocistronically from the opposite strand of DNA. Both transcription units are probably terminated at a 25 bp inverted repeat 3 and 5 bp downstream of the stop codons of the apcL9.5 and cpcL29 genes, respectively. The levels of both transcripts are greatly reduced in the dark as is the psbA transcript. Downstream from the phycobiliprotein gene cluster two open reading frames (ORFs) were found which are homologous to ORFs from plastid DNAs and cyanelle DNA of Cyanophora paradoxa. Sequence homologies between genes analysed in this study and corresponding genes from cyanobacteria, chlorophytic plastids and cyanelles point to a large phylogenetic distance between the plastids of Cyanidium and cyanobacteria and other plastid types.