Between April 14 and 22, 2009, the Aquatic Resources Authority (ARAP) and the National Environment Authority (ANAM) reported the occurrence of massive fish kill between Puerto San Antonio (17N 0475382-UTM 0867216) and the mouth of San Pablo River (17N 0478225-UTM 0867609) in the gulf of Montijo. The affected areas were Membrillal and Palitos Island, Pajarón, Pixbae and Zurrones (Fig.1). Affected fishes were mostly corvine and sardine. Samples of water were collected by local fishermen and transported for identification to the biology laboratories of the University of Panamá at Veraguas. No scientific criterion was applied to the collecting and transport of samples. Samples revealed the presence of Peridinium quinquecorne Abé on floating detritus.