Microcystins are cyclic heptapeptides, composed of five permanently occurring and two variable amino acids. The toxicity of microcystins is associated to the inhibition of the serie/threonine protein phosphatases type 1 (PP1) and 2A (PP2A), enzymem that play an important role in the dephosphorylation of intracellular proteins. Thein inhibitionh in the liver implies hyperphosphorylation of the cytoskeletal filaments, hepatocyte determination and subsequent tumor formation. In order to assure the water quality and the public health, the WHO (World Health Organization) has recommended a maximum level of 1 µg/L of most commonly fund microcystin-LR in drinking water. The strong toxicity of microcystins makes necessary the development of fast, sensitive and reliable methods to detect them. The enzymatic approach based on the PP inhibition informs about the toxicology and is highly sensitive. Electrochemistry has been widely used in biosensors because but it has not been exploited neither for the detection of protein phosphatase inhibition by microcystins. We propose new strategy for the electrochemical monitoring of microcystin in water. The enzymatic approach is based on the inhibition of protein phosphatase (type 2A) by microcystins. The enzyme has been immobilised on the screen-printed working electrode surface by encapsulation with polyvinyl alcohol bearing styrylpyridium groups (PVA-SbQ). Immobilisation conditions have been optimised by colorimetry. The electrochemical transduction has been achived by using a new designed enzyme substrate, catechyl monophosphate, wich is electrochemically active only after dephosphorylation by the non-inhibited enzyme. This bioelectrochemical sensing strategy have led to development of amperometric biosensors for water quality assessment.