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A survey conducted between June 2000 and May 2002 on the island of Tutuila, American Samoa, recorded filamentous green algae of the order Trentepohliales (Chlorophyta) and their plant hosts. Putative pathogenicity of the parasitic genus Cephaleuros and its lichenized state, Strigula, was also investigated. Three genera and nine species were identified: Cephaleuros (five spp.), Phycopeltis (two spp.), and Stomatochroon (two spp.). A widely distributed species of Trentepohlia was not classified. These algae occurred on 146 plant species and cultivars in 101 genera and 48 families; 90% of the hosts were dicotyledonous plants. Cephaleuros spp. have aroused worldwide curiosity, confusion, and concern for over a century. Their hyphaelike filaments, sporangiophores, and associated plant damage have led unsuspecting plant pathologists to misidentify them as fungi, and some phycologists question their parasitic ability. Of the five species of Cephaleuros identified, C. virescens was the most prevalent, followed by C. parasiticus. Leaf tissue beneath thalli of Cephaleuros spp. on 124 different hosts was dissected with a scalpel and depth of necrosis evaluated using a fourpoint scale. No injury was observed beneath thalli on 6% of the hosts, but fullthickness necrosis occurred on leaves of 43% of hosts. Tissue damage beneath nonlichenized Cephaleuros thalli was equal to or greater than damage beneath lichenized thalli (Strigula elegans). In spite of moderate to severe leaf necrosis caused by Cephaleuros spp., damage was usually confined to older leaves near the base of plants. Unhealthy, crowded, poorly maintained plants tended to have the highest percentage of leaf surface area affected by Trentepohliales. Parasitic algae currently are not a problem in American Samoa because few crops are affected and premature leaf abscission or stem dieback rarely occur.