Abstract: Hymenomonas carterae is an unicellular marine alga,which is spheroid and is covered with one layer of coccoliths;two flagella are slightly different in length and attach to the front end of the cell.The length of the flagella is about 1.5 times as the cell diameter.When the cells are cultured in MES Ⅲ medium,under temperature 25±1℃,light intersity 2000 lx,light dark period 14:10(L:D)hours,it was found that the cells swim actively with their flagella in nomal length in the light and sink to the bottom with their flagella withdrawn into cells in the dark period.Further experiments reveal:1.Light stimulates the flagella to stretch and dark stimulates the flagella to withdraw.2.Flagellar stretch and withdrawal correspond strictly with the light-dark cycle.Flagella stretch out 20 minutes after the beginning of light period and withdraw after the beginning of dark period.3.The phenomenon of cyclically flagellar stretch and withdrawal disappears under continuous light,Obviously,the cyclically flagellar stretch and withdrawal of Hymenomonas cartearis is a kind of exogenous rhythm which is regulated by the light-dark cycle,the rhythm of algal flagellar stretch and withdrawal is reported for the first time.