Photosensitized extracts of 28 seaweed species were tested in vitro to examine the variation in their algicidal activities against the toxic microalga Cochlodinium polykrikoides. When both seaweed extracts and microalgae were exposed to UV and visible light, methanol extracts of Porphyra yezoensis showed 5.7- and 4.4-fold increasesin light-dependent algicidal activity, respectively. When only seaweed extracts were exposed to both UV (365 nm) and visible light (white lamp) at the same time, methanol extracts of Enteromorpha linza and Carpopeltis affinis showed 3.3- and 3.4-fold increases in algicidal activity, respectively. When UV-photosensitized extracts were left in the dark, the algicidal activity of Ecklonia cava increased 13-fold after 5 h. When visible light-photosensitized extracts were left in the dark, the algicidal activity of Monostroma nitidum increased by 3.3-fold in 1 h.