In this paper, we describe a protocol to obtain a site-directed mutants in the psbA gene of Chlamydomonas reinhardtii, which overcomes several drawbacks of previous protocols, and makes it possible to generate a mutant within a month. Since the large size of the gene, and the presence of four large introns has made molecular genetics of the psbA gene rather unwieldy, we have spliced all of the exons of the psbA gene by PCR to facilitate genetic manipulation and sequencing of the gene. The resultant construct (plasmid pBA153, with several unique restriction sites introduced at exon boundaries) carried 1.2 and 1.8 kb intact sequences from the 5'- and 3'-flanking regions, respectively. The plasmid was used to transform a D1-deletion mutant and was found to complement the deletion and restore photosynthetic activity. In addition, a bacterial aadA gene conferring spectinomycin resistance (sper) was inserted downstream of the intron-free psbA gene, to give construct pBA155. This allowed selection of mutant strains deficient in photosynthesis by using spectinomycin resistance, and eliminated the possibility of selection for revertant strains which is a consequence of having to use photosynthetic activity as a selection pressure. Finally, pBA155 was used to construct pBA157, in which additional restriction sites were inserted to facilitate cassette mutagenesis for generation of mutations in spans thought to be involved in donor-side interactions. All psbA deletion strains transformed with intron-free psbA-aadA constructs encoding the wild-type D1 sequence, and screened on spectinomycin plates for the sper phenotype, were able to grow photosynthetically, and all showed identical kinetics for electron transfer from primary (QA) to secondary quinone (QB) in photosystem II, as assayed by the decay of the high fluorescence yield on oxidation of the reduced primary acceptor (QA-).