Five species of the lichen genus Porimi are determined among the collection made during an expedition to the Mariana Islands, Micronesia in 1992. P. aggregata P. M. McCarthy et Harada sp. nov, is described as new. The following four species are reported as new to Mariana Islands: P. cestrensis (Mich.) Muell. Arg., P. mastoidea (Ach.) Muell. Arg., P. papuensis P. M. McCarthy and P. tetracerae (Ach. in Ach.) Muell. Arg. A description and figure are provided on the basis of those specimens. A key to the species is presented. lichens, Trichotheliaceae, Porina, Porina aggregata P. M. McCarthy et Harada sp. nov., Micronesia, Mariana Islands, flora