Two marine, heterotrophic, aerobic, yellow-pigmented, agarolytic bacterial strains that are motile by means of gliding were isolated from the green alga Acrosiphonia sonderi and from sea water. Comparative 16S rRNA gene sequence analysis revealed an affiliation between the strains studied and the genus Formosa, a member of the family Flavobacteriaceae. The level of sequence similarity between strain KMM 3901T and Formosa algae KMM 3553T was 99.1 %. The results of DNA-DNA hybridization experiments and phenotypic analysis indicated that the strains represent a novel species of the genus Formosa, for which the name Formosa agariphila sp. nov. is proposed, with KMM 3901T (= KCTC 12365T = LMG 23005T = DSM 15362T) as the type strain. The description of the genus Formosa is emended with newly obtained data.