Polyurethane sponges were suspended in the surface waters of a Mecklenburg County, North Carolina pond that was posted and unused, a Cheboygan County, Michigan "acid" bog pond and an "alkaline" gravel-pit pond, and a Montgomery County, Virginia farm pond. The pH of the sponge water was often substantially different from that of the surrounding water (e.g. sponge water pH 6.8, pond water pH 10.5). These differences are attributed to the microbial communities inhabiting the sponges. If this is generally true for microbial growths associated with substrates, the relationship of these species to the quality of the surrounding water should be re-examined.