The present work addressed the study an episode of red waters of great extent, which was observed off the Oum Er Rbia estuary in spring 2019. The responsible species was Prorocentrum micans. The study was carried out on three stations with different characteristics, distributed in the estuary upstream and downstream of an urban wastewater discharge. The cell densities observed were maximum in March 2019. Some environmental parameters (temperature, salinity, dissolved oxygen, chlorophyll (a) and pheopigments) were simultaneously recorded. The P. micans densities increased from downstream to upstream with cell concentrations of 1.03×10^5 Cell/l in S3, 9.1×10^6 Cell/l in S2 and 14.11×10^7 Cell/l in S1. The contribution of P. micans to the total dinoflagellate density was higher and reached 99.54% in station S1. These blooms would be associated with high concentrations of chlorophyll (a) and pheopigments, which equaled 40 µg/l and 909 µg/l, respectively, and dissolved oxygen supersaturations (17.05 mg/l). This phenomenon also corresponded to a marked decrease in salinity. According to the application of the Kruskal-Wallis test, the difference was found to be significant between the three stations (p = 0.0034). In addition, densities were higher in surface water than in bottom water at all three stations. However, this difference did not appear to be statistically significant (p > 0.05).