The chloroplast genomes of Chlamydomonas spp. contain introns that belong to the two major classes found in organelles: group I and group II. Some of the members of both classes are ribozymes capable of self-splicing in vitro and, indeed, most of the group I introns studied in Chlamydomonas spp. are autocatalyic. The biochemical characterization of these introns suggests that, in vivo, their splicing must be dependent on protein factors. It is particularly interesting in this respect that light regulates the splicing of the four group I introns in the psbA gene of C. reinhardtii. The two group II introns of the psaA gene are split into two or more pieces which are transcribed separately from different chloroplast loci. The mature psaA RNA is assembled from the separate precursors in two steps of intermolecular trans-splicing. There are many nuclear mutants defective in psaA mRNA maturation, which identify a large number of protein factors that are required, directly or indirectly, for trans-splicing.