Brown algal swarmers usually exhibit positive or negative phototaxis. Such behaviors influence the increasing or decreasing dispersal distance or colonization on the new substratum. We confirmed that the sign of phototaxis (negative or positive) in male gametes of Mutimo cylindricus was affected by extracellular Ca2+ influx through Ca2+ channels. Under the control condition (10(-2) m [Ca2+]), male gametes swimming with a helical rotation of their cell body mostly showed positive phototaxis. At 10(-3) m [Ca2+], more than half of the male gametes showed positive phototaxis, whereas the others showed negative phototaxis. From 10(-4)-10(-5) m [Ca2+], the phototactic sign changed to negative. When these negative phototactic gametes were transferred back to the control condition, the phototactic sign reverted to positive. At 10(-6) m [Ca2+], some of male gametes showed negative phototaxis, but most showed no phototaxis or flagellar beating. Lanthanum, a Ca2+ channel blocker, affected the sign of phototaxis at 10(-4) m [La3+] under 10(-2) m [Ca2+], and male gametes mostly showed negative phototaxis. A further increase in [La3+] inhibited phototaxis and flagellar beating. These results pointed out the involvement of Ca2+ channels that were blocked by La3+ in phototaxis and flagellar beating.