The composition of phytoplankton in the Ebro River estuary (Spain) was analyzed at six sampling stations in April, July and October 1999 and February 2000, based on plankton net and bottle samples. A total of 304 taxa belonging to 13 classes were identified. Bacillariophyceae and Chlorophyceae were the most important groups. The diatom genera Nitzschia and Navicula provided most species (25 and 17, respectively). Thalassiosira duostra is recorded for the first time for this ecosystem. We propose the following new combinations: Monactinus simplex (Meyen) Corda var. echinulatum (Wittrock) comb. nov. and Monactinus simplex (Meyen) Corda var. sturmii (Reinsch) comb. nov. Keywords: phytoplankton, estuary, taxonomy, Ebro, Spain.