Cyanobacteria are known to produce hepatotoxins, the functional and ecological role of these toxins, however, remains largely unclear. The toxic properties of cyanobacteria collected in Antarctica were investigated to determine whether toxin-producing species can also be found under these environmental conditions. Samples were collected from ponds near Bratina Island on the McMurdo Ice Shelf, Antarctica in the summers of 1997 through 1999. These ponds form from melt-water in the Antarctic summer and are colonized by benthic cyanobacterial mats. Oscillatoriales, Nodularia sp., and Nostoc sp. constituted the major taxa in freshwater ponds, while Nostoc sp. was missing from brackish and saline ponds, as determined by light microscopy. Samples were taken from either floating, submerged or benthic mats, subsequently frozen, freeze-dried, and extracted for in vitro toxicity testing. The phosphatase-inhibiting activity of the extracts was tested using rape seed phosphatase and 32P-phosphorylase a as substrate with microcystin-LR as the standard. The cytotoxic properties of the extracts were investigated using rainbow trout hepatocytes determining MTT metabolism and trypan blue dye exclusion. The results show that all cyanobacterial extracts have some phosphatase-inhibiting activity, of which approximately half had significantly greater than 50% inhibiting activity. Cytotoxic properties, apparently independent of the phosphatase inhibiting activity, were also detected. Toxic strains of cyanobacteria can therefore also be found in Antarctica and this finding may lead to further insight into potential ecological roles of cyanobacterial phosphatase inhibiting toxins.