The unicellular alga Chlamydomonas reinhardtii exhibits active orientation with respect to a chemical gradient. As already reported before, Chlamydomonas cells are attracted to ammonium ion and some sugars. Chemotactic orientation of C. reinhardtii is Ca₂₊ dependent. The Ca²⁺ channel blockers Cd²⁺, La³⁺, ruthenium red, verapamil and diltiazem completely inhibit chemotaxis. These data indicate that the Ca²⁺ currents mediating chemotactic responses of C. reinhardtii are carried by L-type Ca²⁺ channels. In contrast to chemosensory signaling in eukaryotic cells possessed amoeboid motility (Dictyostelium discoideum amoebae, leucocytes), chemosensory signaling in swimming cells of C. reinhardtii does not involve inositol 1,4,5-trisphosphate. Our results suggest that PTX2, that is a component of the signal transduction pathway for photoshock and phototaxis, may well be involved in chemotaxis to sugars.