Many free-swimming unicellular organisms show negative gravitaxis, i.e. tend to swim upward, although their specific densities are higher than the medium density. To obtain clues to the mechanism of this behavior, we examined how a mutation in motility or behavior affects the gravitaxis in Chlamydomonas. A phototaxis mutant, ptx3, deficient in membrane excitability showed weakened gravitaxis, whereas another phototaxis mutant, ptx1, deficient in regulation of flagellar dominance displayed normal gravitaxis. Two mutants that swim backwards only, mbo1 and mbo2, did not show any clear gravitaxis. We also isolated two novel mutants deficient in gravitaxis, gtx1 and gtx2. These mutants displayed normal motility and physical characteristics of cell body as assessed by the behavior of anesthetized cells. However, these cells were found to have defects in physiological responses involving membrane excitation. These observations are consistent with the idea that the gravitaxis in Chlamydomonas involves a physiological signal transduction system, which is at least partially independent of the system used for phototaxis.