The chloroplast psbB, psbF, petB, and petD genes are cotranscribed and give rise to many overlapping RNAs. The mechanism and significance of this mode of expression are of interest, particularly because the accumulation of the psb and pet gene products respond differently to both light and, in C4 species such as maize, developmental signals. We present an analysis of the maize psbB, psbF, petB, and petD genes and intergenic regions. The genes are organized similarly in maize (a C4 species) and in several C3 species. Functional class II-like introns interrupt the 5' ends of petB and petD. Both spliced and unspliced RNAs accumulate; these encode alternative forms of the petB and petD proteins, differing at their N-termini. Promoter-like elements between psbF and petB, and biased codon usage suggest that the differential regulation of the psb and pet genes might be achieved at both the transcriptional and translational levels.