Static-renewal ecotoxicity trials monitored growth of Hydrilla verticillata in conjunction with exposure to Cylindrospermopsis raciborskii whole-cell extracts containing the cyanotoxin cylindrospermopsin (CYN). Maximum exposure concentrations were 400 micorgrams per litre CYNover 14 days. The responses of Hydrilla to the treatments were variable according to the toxin concentrations and lengths of exposure. Plant deaths, chlorosis, and necrosis were not recorded from treated plants. However, Hydrilla experienced significant growth stimulation and redistribution of plant resources in conjunction with exposure to the whole-cell extracts. Root production was particularly impacted. The results of this study imply that root production could aid in reducing C. raciborskii cell concentrations and CYN toxicity. Results of chlorophyllanalyses differed, indicating that CYN in whole-cell extracts might exert complex effects on photosynthesis.This is the first study to describe the responses of an aquatic macrophyte following exposure to C. raciborskii whole-cell extracts containing the cyanotoxin cylindrospermopsin.