The chloroplast chromosome of spinach (Spinacia oleracea) is a double-stranded circular DNA molecule of 150,725 nucleotide pairs. A comparison of this chromosome with those of the three other autotrophic dicotyledons for which complete DNA sequences of plastid chromosomes are available confirms a conserved overall structure. Three classes of open reading frames were distinguished: (1) genes of known function which include 108 unique loci, (2) three hypothetical chloroplast reading frames (ycfs) that are highly conserved interspecifically, and (3) species-specific or rapidly diverging 'open reading frames'. A detailed transcript study of one of the latter (ycf15) shows that these loci may be transcribed, but do not constitute protein-coding genes.