Historically, Illinois had as many as eighty mussel species in its rich network of streams and lakes. However, recent stream surveys in Illinois and throughout the United States have documented drastic declines in mussel populations. Today mussels may be the most endangered group of animals in North America. Of the eighty species that once lived in Illinois waters, only fifty-nine have been found in the state since 1970. Eleven of the remaining fifty-nine species are now known from only a single river system or population, and they may disappear by the end of the decade. Many factors are responsible for the decline of freshwater mussels, including siltation, pollution, loss of habitat, channelization of streams, and competition from exotic species like the zebra mussel. Illustrations: Above: Bar chart of percent decline of species diversity in selected Illinois streams (data from Illinois Natural History Survey. Right: Base map of streams in Illinois (from Smith 1979: xvi). Below: Colony of zebra mussels attached to a pink heelsplitter (photo by Kevin S. Cummings).