Over the last twenty years, cyanobacteria have become the predominant phytoplankton in La Albufera de Valencia (coastal lagoon). Dense populations of Planktothrix (Oscillatoria) have been observed, sometimes in excess of 3.10 5 individuals/ml. This genus has been commonly found to produce microcystins, a hepatotoxin. In order to assess the presence or absence of these toxins in la Albufera, samples were taken in July and August of 1999. Physico-chemical and biological parameters were assessed by standard techniques and microcystin concentrations were measured with an E.L.I.S.A. Concentrations of microcystin were found to range from 5.0 to 2102.9 ng/l (average 300.1 ng/l). Microcystin levels appeared unrelated to the abundance of known microcystin producers (i.e Planktothrix, Microcystis), but were generally higher when Planktolyngbya contorta was abundant and lower during Chroococcus dispersus dominance. This study constitutes the first report of microcystins in Spain.