In 2019/2020, 23 sampling campaigns (one campaign per month) were carried out in two Moroccan stations at the mouths of Bouregreg (Rabat) and Sebu (Mehdia). The aim of this work is to study the biodiversity, abundance and ecology of marine plankton (phytoplankton). Three taxa were identified at the two stations such as Alexandrium sp; Dinophisis sp and Gymnodinum sp. At the Mehdia site, low densities are recorded during the winter and this for the three taxa, however these densities are maximum in spring and at the end of summer and beginning of autumn for Dinphisis with two peaks recorded in May (450 cells / ml) and in September (265 cells / ml). At the Rabat site these densities are maximum in autumn and summer for Gymnodinum with respectively rates of 860 cells / ml displayed in October and 850 cells / ml in August. On the other hand, the monthly distribution of Aexandrium shows that the peaks are recorded during the warm months (May = 165 cells / ml; August = 170 cells / ml and the month September = 160 cells / ml). The results of the physicochemical analyzes show that despite the somewhat high levels of phosphates and nitrates in the Mehdia site, the taxa therefore tolerate these levels. On the other hand, salinity and temperature represent two conditioning parameters of the growth of these phytoplankton. Taking into account the degree of abundance in the two sites of these taxa which are qualified toxic, the persons in charge must redouble their efforts to bring a biological balance with the other animal classes.