Two new genera of Upper Silurian stromatoporoids in order Actinostromatida are described. Genus Bicolumnostratum Stock, with type species B. micum (Bogoyavlenskaya), is characterized by two kinds of pillars and nonaligned colliculi, and is assigned to family Actinostromatidae. Genus Acosmostroma Stock, with type species A. ataxium Stock new species, contains irregular micropillars and microcolliculi, and is assigned to family Densastromatidae. Two additional new species are Acosmostroma glascoense Stock and A.? cobleskillense Stock. A fourth species is A. tenuissimum (Parks). Bicolumnostratum is known from Ludlow- and Pridoli-age strata, whereas the occurrences of Acosmostroma are strictly Pridoli in age.