A comparative study was conducted to examine differences and similarities between the benthic fauna of a naturally occurring rocky shoal habitat in the northern portion of Lake Michigan, Waugoshance Point, and the fauna associated with a man-made rocky riprap site at the Donald C. Cook Nuclear Power Plant in the southeastern part of the lake. Concrete substrate samplers were used as a method to quantify collections. The man-made riprap site and the rocky shoal site shared many of the same group of organisms, but differences between the sites were apparent, particularly in structure of the communities. The fauna of Waugoshance Point was dominated by filter-feeding organisms, whereas predators were most abundant at the Cook riprap location. Presence of Cladophora on the riprap is shown to be one of the most important factors accounting for the difference in densities and relative abundances of the fauna between the two sites. The Cladophora population created a habitat favorable to many types of benthic organisms, and their numbers dropped sharply with its disappearance from the riprap in late fall. The artificial substrates at Cook were colonized by species significantly different from the Ponar samples taken in adjacent unconsoliated sandy substrates, indicating the extent of change produced by the riprap in the otherwise featureless bottom.