The mechanisms governing chemotaxis inChlamydomonas reinhardtiiare largely unknown compared to those regulating phototaxis despite equal importance on the migratory response in the ciliated microalga. To study chemotaxis, we made a simple modification to a conventional Petri dish assay. Using the assay, a novel mechanism governingChlamydomonasammonium chemotaxis was revealed. First, we found that light exposure enhances the chemotactic response of wild-typeChlamydomonasstrains, yet phototaxis-incompetent mutant strains,eye3-2andptx1, exhibit normal chemotaxis. This suggests thatChlamydomonastransduces the light signal pathway in chemotaxis differently from that in phototaxis. Second, we found thatChlamydomonascollectively migrate during chemotaxis but not phototaxis. Collective migration during chemotaxis is not clearly observed when the assay is conducted in the dark. Third, theChlamydomonasstrain CC-124 carryingagg1−, theAGGREGATE1gene (AGG1) null mutation, exhibited a more robust collective migratory response than strains carrying the wild-typeAGG1gene. The expression of a recombinant AGG1 protein in the CC-124 strain suppressed this collective migration during chemotaxis. Altogether, these findings suggest a unique mechanism; ammonium chemotaxis inChlamydomonasis mainly driven by collective cell migration. Furthermore, it is proposed that collective migration is enhanced by light and suppressed by the AGG1 protein.