A first report of the coccolithophorid flora in the phytoplankton of the Istanbul Strait is presented in this paper. Samples were taken with a Nansen bottle at three depths (surface, -5 in, 10 m), on the coast of Istanbul Strait, considering four different areas: Uskudar, Cengelkoy, Kanlica and Beykoz during May 97 and August 1998. Density, biomass and total phytoplankton chlorophyll a content were calculated. During the study period, some physical and chemical parameters were measured. A total of five species of coccolithophorids were observed at sites, however considerable differences were observed in the seasonal fluctuations of density and biomass. A bloom was observed during May 1997. Calyptrosphaera species was the most dominated species followed by Anacanthoica acanthos (Shiller) Deflandre. Biomass, quantified as biovolume, showed a well-defined seasonal pattern.