Three species of trepostome bryozoans formed syn vivo associations with the Cornulites in the Late Ordovician of Estonia. Cornulites sp. and Mesotrypa excentrica presumably formed a true symbiotic association. This is the first known case of symbiosis between cornulitids and bryozoans. It is not known whether this symbiotic association was obligatory of facultative for the cornulitid, but it was facultative for the bryozoan. In this association cornulitids may have competed for the food with bryozoans and the association may have been parasitic. The remaining associations between cornulitids and bryozoans were accidental. Most common skeletonized endobionts of the Ordovician bryozoans were not cornulitids, but conulariids and rugosans.