Three ichnospecies of the 'small round hole' Oichnus Bromley occur in tests of the large holasteroid echinoid Hemipneustes striatoradiatus (Leske) in the type area of the Maastrichtian (Upper Cretaceous) in The Netherlands and Belgium; Oichnus simplex Bromley (penetrative), Oichnus paraboloides Bromley (non-penetrative and showing two distinct morphologies) and Oichnus isp. nov. (non-penetrative). The two distinct morphologies of O. paraboloides (both shallow, one with a central boss) are gregarious, but do not occur together on the same specimens, suggesting they were generated by different taxa. Oichnus paraboloides with a central boss occurs on H. striatoradiatus from the upper Nekum Member, Maastricht Formation. Tests of the host echinoid are smaller in the overlying Meerssen Member, Maastricht Formation, where they are infested by Oichnus isp. nov., the largest borings found in these echinoids. Oichnus isp. nov. has concave walls and a large central boss. Blisters inside tests from the Meerssen Member show that this infestation occurred when the echinoid was alive. It is more likely that the borer gained benefit from the protection, elevation and mobility provided by the echinoid test, rather than followed a parasitic habit. It is postulated that producers of these borings in H. striatoradiatus may have been genetically related and increased in size during the Maastrichtian even as the host echinoids showed a size decrease. This size increase in H. striatoradiatus was genetic and cannot be related to increase in size of borings.