Chlamydomonas has long been a favourite organism for genetic and biochemical studies of flagellar motility and assembly, photosynthesis, and organelle genomes. With the recent development of procedures for the efficient transformation of its nuclear genome, Chlamydomonas has become accessible to a wide range of molecular genetic approaches, including gene tagging by insertional mutagenesis and cloning by complementation. The availability of these powerful techniques is stimulating interest in Chlamydomonas as a model system for research in areas where it previously has not been widely exploited. One such area that holds particular promise is phototransduction and the behavioural response to light.