The description of seasonal variation of zooplankton in acuatic ecosystems is useful to define movements of the mass of water and to stimulate the dynamics of fisheries resources in areas such as Golfo de Montijo and which represents one of the main halieutics zone of the country. Superficial horizontal dragging was done using planktonic net of 243 ?m opening for five minutes at a speed of two knots equipped with a flow-meter. Samples were taken during an annual cycle from May (2009) to April (2010), and showed dominance of calanoids copepods (75,4%), larves of crustaceans decapods (8,6%) and cirripedia (4,5%), chaetognatha and small eggs of fishes (2,3%). Copepods are the holoplanktonic representative most important in all planktonic studies and along with other organisms, formed part of aquatic secondary producers. The productive secondary index was determined taking in consideration the total of individuals and the volume (m3) of water filtered by the net. This showed progressive increase from June (2009) to February (2010), preceded by a high production in the May (2009).