The abundance of the native snail Pyrgulopsis weakly increased (p = 0.07) with abundance of the invasive snail, Potamopyrgus (a) and the native snail Physa moderately decreased (p = 0.02) with abundance of Potamopyrgus (b). Although the relationship between the abundance of Potamopyrgus and Physa appears to be strongly influenced by the two sites where the abundance of Potamopyrgus was highest (>500 individuals/sample), the evidence for a negative relationship is much stronger when the two extreme values are omitted.