There have been several extensive studies during the past few years to establish general design criteria of nitrifying trickling filters (NTFs).1-3 Several conclusions reached by Gullicks and Cleasby, however, appear to dispute these criteria. For example, Gullicks and Cleasby imply that influent and bulk dissolved oxygen concentrations are more important than organic loads. In addition, they neglect more recent work emphasizing the critical role of thick biofilms in maintaining high nitrification rates in NTFs.3 We are concerned that casual review and acceptance of their study could lead to less efficient design of NTFs and believe several aspects of their analysis must be critically examined.