Cylindrospermopsin (CYN) is a protein-synthesis inhibitor produced by several species of blue-green algae, most notably, Cylindrospermopsis raciborskii. Despite a global increase in toxin-producing C. raciborskii blooms, the ecological effectsof CYN exposure, and the potential for CYN to accumulate in aquatic organisms, have been poorly studied. In this study, duckweed plants (Spirodela oligorrhiza) were examined during seven-day exposures to C. raciborskii whole cell extracts containing environmentally relevant CYN concentrations (maximum 120 migrograms per litre). Both stimulation and inhibition of growth were recorded from the duckweed; with considerable variability in responses depending on treatment concentrations and the length of incubation. S. oligorrhiza chlorophyll content was also affected, with small peaks in chlorophyll a recorded in conjunction with exposure to treatments containing 8 migrograms per litre CYN. Bioconcentration of CYN was not recorded during separate studies examining six or twelve days’ exposure to C. raciborskii whole cell extracts containing 0 – 500 migrograms per litre CYN. Whilst nanogram quantities of toxin were recovered from plant tissues, these were considered likely to represent toxin adsorbed to the plant cell walls.