Synopsis Blooms of the cyanobacterium Cylindrospermopsis raciborskii have become a reoccurring feature of our waterways and storages worldwide. These blooms have the potential to threaten the health of both humans and animals through the contamination of recreational and drinking water supplies. C. raciborskii has the potential to produce the cyclic alkaloid cylindrospermopsin (CYN). This cyanotoxin was believed to have been the causative agent in the Palm Island "Mystery Disease" poisoning incident in Queensland in 1979, where 148 people were afflicted with an illness (Bourke et al., 1983) (Hawkins et al., 1985). Although no further reports of human poisonings have been attributed to CYN, a multitude of livestock deaths have been ascribed to this toxin (Saker et al., 1999) (Thomas et al., 1998). An understanding of the mechanisms that determine CYN's fate in the environment have important implications for authorities managing waterways. A number of mechanisms have been investigated or discussed in relation to other cyanotoxins, such as the microcystins, and it has been suggested that microbial degradation was responsible for the greater part of microcystin removal in aquatic systems.This study represents the first detailed investigation into the microbial degradation of CYN and the isolation and characterization of bacteria with CYN degradative ability. Investigations also assessed the feasibility of using biodegradation in a slow sand filtration (SSF) pilot plant to remove soluble CYN from a contaminated water body with the aim of producing safe water to needy communities. Natural aquatic organisms are capable of degrading CYN, in the concentration range of 0 to 950pg L-1 once the CYN has been released from the cyanobacterial cells. Removal of the CYN appears to be dependent on either the presence of C.raciborskii cells themselves or/and a history of toxic C. raciborskii blooms. Waterbodies with no history of toxic C. raciborskii blooms or any C. raciborskii cells present at the time of the study did not show CYN degradative activity over a 40 day experimental period. In the waterbodies where CYN degradative activity was present, a history of toxic C. raciborskii blooms was noted. In some cases loss of CYN was delayed (i.e. acclimation period) until the endemic population of bacteria developed the necessary enzymes for CYN degradation or/and the biomass of organisms reached a level that loss of CYN was detectable. However, it has been shown that repeated exposure to CYN resulted in a shorter acclimation period prior to the commencement of detectable loss of CYN. A number of CYN degradative bacteria were isolated from Hervey Bay Retardation Basin (HBRB), a water body with a history of toxic C. raciborskii blooms. One of the isolated bacteria was identified as Delftia sp. by direct sequencing of its 16S rRNA gene. This organism belongs to the Comamondaceae family (Wen et al., 1999) which belongs to the p subclass of the Proteobacteria (Stackebrandt et al., 1988). The type species D. acidovorans has been shown to degrade a range of herbicides, phosphorus and analine based compounds (Boon et al., 2001) (Mueller and Babel, 2001). The ability of this organism to degrade these chemicals suggests that wide ranges of enzymatic processes are present and so the isolation of a strain capable of degrading CYN is not a surprising result. A filtration plant consisting of a biologically active roughing filter (RF) and slow sand filter (SSF) was effective in removing 100% of C. raciborskii cells and soluble CYN from a natural occurring waterbody. The cell density and the soluble CYN concentration during this study ranged from 5860 to 76 000 cells mL-1 and 0 to 2.2 µg L-1 respectively. These results clearly demonstrated the potential for using biodegradation as means of producing water free from soluble CYN. Many factors influence the CYN degradative capability of a waterbody. The results obtained from this study provide water authorities managing both recreational and drinking water supplies with valuable information allowing them to make informed decisions relating to withholding periods following toxic C. raciborskii blooms.