From study of the Devonian "recifs rouges" of Belgium and the Silurian mounds of the Quebec Appalachians, we propose that deposition of the red stromatactis limestone facies of Cambrian-Devonian carbonate mounds was controlled by sponges and that the red color and stromatactis result from early diagenesis within a few meters below the substrate-water interface in the deep-marine environment. The common presence of sponge body fossils, as well as various stages of sponge preservation, ranging from easily delineated to indiscernible bodies, in the Belgian "recifs rouges" support the conclusion that a large part of the red finely crystalline limestone originated from early cementation of sponge communities or spicule-rich organic mats derived from degradation of sponge communities. The role of bacteria and other microbes, although widely advocated as the main primary builders of carbonate mounds, is difficult to assess. Microbial communities may have contributed to mound accretion as primary builders and/or mud producers, but their presence cannot be documented. We suggest that they may have acted as agents for concurrent sponge putrefaction and early cementation (biodiagenesis) during the sulfate-reduction phase under alkaline pH and anoxic conditions. We interpret stromatactis as a spar body that resulted from early marine cementation of a cavity network created by excavation of uncemented material in partly indurated, decaying sponges and spicule-rich organic mats derived from degradation of sponge communities through circulation of interstitial water in the uppermost few meters of the sediment. This cavity network was connected to sea floor and flushed by oxic waters that changed the diagenetic environment from anoxic to oxic, converting amorphous iron sulfide to hematite and giving the facies its red color. Recognition of the dominant role of sponge communities in the construction of red stromatactis limestone mounds of the Cambrian-Devonian time interval has important implications for the geological record. This community was the main deep-water mound-building community during this time, but is difficult to recognize because of its variable but often poor preservation.