Motile cells of unicellular green alga Chlamydomonas reinhardtii can perceive both light and chemical signals. Algal mutants with defects in the dynein arm structure (ida4 and oda22), ptx mutants deficient in the axonemal sensitivity to calcium ions, and ppr mutants lacking an ionic channel specific to the photophobic response were analyzed. The mechanism of cell orientation in the chemotactic responses was shown to depend on functional differences between cis-and frans-flagella, which differ in the beat frequencies and sensitivity of their axonetnes to submicromolar calcium concentrations. Thus, the mechanisms controlling flagellum operation are similar in the case of chemotaxis and phototaxis but distinct from that for the photophobic response.