Chemotaxis of Chlamydomonas reinhardtii 137c cells towards maltose and sucrose was observed by capillary assay. The threshold concentration was approximately 10(-5) M for maltose and 10(-3) M for sucrose. The peak accumulation of cells occurred at 10(-3) M for maltose and 10(-2) M for sucrose. A selection procedure for chemotaxis mutants was developed. Mutant strain CHE-1 was not attracted by maltose; mutant strain CHE-2 was not attracted by sucrose. Addition of attractant fully inhibited photoaccumulation of cells. After a period of time the photoresponse of cells recovered. Under the conditions of our experiments the period of adaptation lasted 15-20 min in wild-type cells and 4-5 min in mutant cells on the given sugar. Glucose and acetate did not attract cells of Chlamydomonas. Added to the medium, these compounds had no effect on the photoaccumulation of cells.