In the Cantabrian Mountains stromatoporoids only have been found up to now in Devonian formations. They occur together with tabulate and rugose corals and brachiopods. Together with these organisms they form biostromes or just biogenetic layers of brecciated and overturned colonies. Four primary microstructures could be distinguished: compact, microlaminate, ordinicellular, and cellular. Alteration seems to begin before fossilization in many cases: the microtissue becomes flocculent by migration and/or destruction of specks. After fossilization the microstructure is altered mainly by migration of specks along slip planes and by rearrangement of the calcite crystals. In this paper the original microstructure is used as the main character for the determination of genera. The form of the coenosteum and features of the gross structure such as superposition of pillars, absence or presence of ring pillars, spacing of laminae and pillars, and others, are strongly influenced by ecological factors. Therefore they cannot be used as characters for the definition of genera and often not even for species. Four genera can be distinguished in the Spanish material: Actinostroma (compact with continuous pillars), Stromatoporella? (microlaminate), Stromatoporella? (ordinicellular) and Stromatopora (cellular). The genera Geronostroma and Atelodictyon are considered to be synonyms of Actinostroma. The genus Stromatoporella should be divided into two genera: one genus with microlaminate and one genus with ordinicellular microstructure. In the microlaminate genus the genera Clathrocoilona, Trupetostroma and Stictostroma (partly) should also be included and perhaps Idiostroma (partly). The ordinicellular genus can be combined with part of the genus Stictostroma and some species of Anostylostroma. The genera Parallelopora and Ferestromatopora are considered to be synonyms of Stromatopora. For the determination of species an attempt is made to establish the variability of the gross structure for each species. As this variability seems to be rather wide, species determinations are only given when sufficient material was available. The following species are described: Actinostroma papillosum (= A. clathratum), Actinostroma verrucosum?, Actinostroma stellulatum, Stromatoporella? granulata? (microlaminate), Stromatoporella? selwyni (ordinicellular), and Stromatopora huepschi?. Stromatopora concentrica