ABSTRACT. The new endosymbiotic cornulitid Cornulites stromatoporoides n. sp. is described here from the Sheinwoodian stromatoporoids of Saaremaa, Estonia. This new species represents the earliest record of cornulitid endosymbiosis. In lower Sheinwoodian open shelf marls, 30.2 % of stromatoporoids are infested by C. stromatoporoides n. sp.; 76.9 % of stromatoporoids in upper Sheinwoodian shoal limestones also had this endosymbiotic relationship. C. stromatoporoides n. sp. has thus far only been found as an endosymbiont in stromatoporoids. There is a range of one to six, averaging 2.4, endosymbiotic Cornulites specimens per infested stromatoporoid in the lower Sheinwoodian. Large stromatoporoids contain more Cornulites endosymbionts than smaller stromatoporoids.