The development of a transformation system in the green unicellular alga Chlamydomonas reinhardii is important for two reasons. First, a large number of nuclear mutants of this organism have been characterized, including auxotrophs, cytoplasmic and chloroplast ribosome mutants, photosynthetic and flagellar mutants. The isolation of wild-type genes from C. reinhardii and from other plants may be possible by complementation with an efficient transformation system. Second, modification and reinsertion of these genes and their regulatory elements into C. reinhardii cells is likely to provide important insights into their function and regulation. Here, we describe the first successful nuclear transformation of C. reinhardii. A plasmid carrying the yeast arg4 locus and a yeast replication origin has been used to transform a C. reinhardii cell wall-deficient, arginine-requiring mutant. In several transformants the yeast gene appears to have integrated stably into the nuclear chromosomes.