Zebra mussel presence and lake water calcium concentrations were compared to determine if a calcium concentration threshold exists which might preclude colonization of zebra mussels in Michigan inland lakes. The shallow littoral zones of nine northern Michigan inland lakes with calcium concentrations near 28.3 mg/L and considered to have had potential zebra mussel introductions through recreational boating were studied. A survey of native unionids was carried out in the lakes for the purpose of establishing a species inventory list for each lake. Zebra mussel densities were determined for lakes where zebra mussels were detected. Water samples were collected and calcium concentrations determined for each study lake. In five softwater lakes (lakes with calcium concentrations <28.3 mg/L), zebra mussels were not found. Zebra mussels had colonized three of four lakes where calcium concentrations were >28.3 mg/L. Five unionid species were found in seven of the nine lakes surveyed. Unionids were present in four of five lakes where calcium concentrations <28.3 mg/L and where zebra mussels were not detected suggesting a possible unionid refuge from zebra mussel induced mortality and/or extirpation in softwater lakes.