Three strains of Ostreopsis ovata were isolated from sea water and algae collected either along the Tyrrhenian or the Adriatic Sea. Evaluation of growth profile of the three O. ovata strains in batch cultures were analyzed at four light intensities (10, 100, 400, 650 µmol m^-2 s^-1). Ostreopsis ovata cell densities increased at all light intensities except at 10 µmol m^-2 s^-1. The three strains showed growth rates with values from 0.24 to 0.56 day. The allelopathic effect of the filtrate of O. ovata culture on growth of Coscinodiscus granii, Prorocentrum minimum and Coolia monotis was studied. Filtrate from culture in exponential and senescent growth phase was considered. The results revealed a weak allelopathic effect on the growth of the three microalgae with an inhibition on the growth of C. monotis and P. minimum.