Blidinje lake is the largest mountain lake in Bosnia and Herzegovina located at 1185 m a.s.l. and it is an integral part of the Nature Park Blidinje founded in 1995. This paper presents the qualitative composition of phytoplankton and phytobenthos of Blidinje lake and the population density of phytoplankton collected at five locations in the coastal region on its surface. The collection of phytoplankton material was carried out over a period of three months in 2008, while phytobenthos was taken once in September 2008. This paper also describes the environmental conditions in Blidinje lake relevant for phytoplankton and phytobenthos growth. The data presented here contributes baseline information concerning biological diversity essential for evaluation of environmental changes in the future. Conservation of karstic lakes is one way to preserve the overall biodiversity of karstic areas.