A life-assemblage population (189 specimens) of hat-shaped (hemispherical) colonies of Prasopora spp. from the lowermost Verulam Formation (Middle Ordovician) at Kirkfield, Ontario contains macroborings referable to Trypanites Magdefrau.Scolecodonts, and other fragmentary microfossils, identify the organisms that made the borings as polychaete annelids. Fifty-two percent of the bryozoan colonies are bored, and some colonies contain as many as 83 macroborings. The polychaetes bored into dead bryozoan colonies, but it appears that the borers were selective in their choice of substrates: they chose large colonies with diameter/height ratios between 3:1 and 3:2. Concentration of high densities of borings on the sides of the colonies indicates that current direction, and probably also sedimentation related to current flow and turbulence over and around the colonies, were important influences on the borers.The tendency for borings to occur in some colonies and not others and (when they are present) to be numerous and clustered on one side of the colony suggests gregarious clumping behaviour.The polychaete worms that produced these Trypanites macroborings, therefore, were both gregarious and highly selective in their choice of substrate. They chose colonies that provided significant elevation above the local soft sediment substrate, and gave a preferred orientation relative to local currents.