Phytoplankton and physical-chemical parameters were investigated for the first time in the only natural hyperhaline marine lakes (salinity >40) along Croatia’s Adriatic coast, Mala Solina and Velika Solina. Two periods were recognized during the one-year investigation: one euhaline-mesotrophic from December to May and one hyperhaline- eutrophic from June to November. Nutrient limitation appears to have been important in defining the lakes’ seasonal phytoplankton composition. Phosphate was most likely limiting from October to December, silicate from January to April, and nitrogen from June to September when nitrate was depleted. Diatoms were most abundant in November to January, when temperature and salinity were low and nitrate and ammonium were high. They collapsed in March when silicate was depleted. Amphora, Navicula, and other naviculoid diatoms were the most frequent genera. Nitzschia longissima was the most abundant species. Dinoflagellate dominance began in June in Mala Solina and in March in Velika Solina. It continued while temperature, salinity, phosphate, and silicate were high. Oxyrrhis marina was the most abundant dinoflagellate (3.2 × 106 cells L-1). Nanophytoplankton was the dominant size fraction. Chroococoid cyanobacteria were most abundant from May to October, reaching 2.9 × 107 cells L-1 in July. Both nanophytoplankton and small microphytoplankton, such as Oxyrrhis, Scrippsiella, and Tetraselmis, were most abundant under hyperhaline, N-depleted conditions. Toxic and harmful taxa (e.g. Alexandrium, Dinophysis), expanding in Mediterranean waters, were not recorded in the lakes.