Mussels colonize cooling water circuits of power stations by attaching themselves to the pipe or conduit walls using byssus threads. Once manually detached, they quickly try to reattach by producing new byssus threads. In many published reports on antifouling bioassays, the test specimens are exposed to the biocide in an unattached state. These mussels, while trying to reattach, are likely to expose themselves more frequently to the toxic compound when compared to firmly attached mussels. The results of the assay, therefore, could vary, depending on the status of the mussels used. In this paper, we test the hypothesis that the status of attachment could influence the toxicity response of mussels and show that byssally attached zebra mussel, Dreissena polymorpha (Pallas), is more resistant to chlorine than unattached ones. An average increase of 27% in the survival time was observed for attached mussels over unattached ones in the chlorine concentration range of 0.25 to 3 mg/L. It is conclusively shown that the increase in sensitivity of the unattached mussels was related to an increase in the byssal activity, quantified presently as the byssogenesis index. The results indicate that future laboratory toxicity experiments involving mussels should be carried out using byssally attached ones.