Variations in phytoplanktonic biomass and density in a natural mussel bed located in Ensenada de Canguas, Sucre State were studied from May to November 1982. Variations in dissolved oxygen, light penetration and temperature were also studied. Samples were collected from surface to a depth of five meters. No stratification of the parameters was observed. Water temperature values varied between 22.0 and 27.0 °C (April and October 1982, respectively; x̅ = 24.19 °C); while concentrations of dissolved oxygen varied from 2.00 to 5.58 ml O2.l-1 (november and october respectively; x̅ = 4.06 ml O2.l-1). Values of light penetration ranged from 2.0 to 6.0 m. It was found that phytoplanktonic biomass varied from undetectable (June 1982) to 4.65 mg clorof. a.m-3 (April 1982). Phytoplanktonic density fluctuated from 3.0 to 12.1 x 102 org. ml-1 (June and April 1982, respectively; x̅ = 6.6 x 102 org. ml-1). Maximum values of phytoplanktonic biomass and density were measured during the same time. 106 microalgal taxa were identified, of which 54 species were diatoms and the rest were dinoflagellates. Diatoms predominated throughout the period of study. No red tides were observed during the study.