ABSTRACT. A large-scale study of Saginaw Bay was initiated in 1990 and continued through 1979 -1980 (STORED were combined with our 1990 and compared to values from the post zebra mussel period (jail 1991, all 1992 and 1993). At stations with high densities of zebra mussels, chlorophyll and total P decreased by 66% and 48%, respectively, and Secchi disk values increased 88%. At outer bay control stations no significant differences were found for chlorophyll or Secchi disk. When parameters were averaged throughout inner Saginaw Bay, zebra mussels caused a 59% and 43% decrease in chlorophyll and in total phosphorus and a 60% increase in Secchi disk transparency. Although zebra mussels significantly altered water quality parameters in the pelagic region of Saginaw Bay, they did not necessarily change system trophic state; rather they altered the spatial partitioning of resources.INDEX WORDS: Zebra mussels, water quality, Saginaw Bay.