Cyanobacteria are common inhabitants of terrestrial and aquatic ecosystems. In the aquatic environments their success is particularly well documented in fresh and brackish waters. References related to the occurrence of these organisms in marine habitats and especially in what concerns to toxicology are scarce. Reports of cyanotoxins in marine habitats are restricted to the warmer temperate and tropical regions. In this study, cyanobacteria strains belonging to the genera Cyanobacterium, Oscillatoria, Synechocystis and Synechococcus were isolated from rocky beaches along the Portuguese coast and were cultured and screened for toxicological properties. The toxicity to mice was observed after intraperitoneal injection of centrifuged and non-centrifuged extracts. The results showed a wide range of responses with strains showing no signs of poisoning and strains producing protracted or slightly protracted toxic effects. Neurotoxic symptoms and effects in liver, kidney, small intestine and lungs are reported. The identity of the toxic compounds is thus far unknown but the implication drawn from the present data is that more than one toxic compound is produced. There was a clear discrepancy between the toxicity of centrifuged and non-centrifuged extracts. In most cases the pellet was toxic indicating that toxicity can be associated with compounds tightly bound to the cell walls.