Egirdir Lake is one of the largest lakes of Turkey. It has provided some of drinking-water for Isparta since 1994 due to the problem of water shortage. A few successive blooms were firstly observed in the western shores of Egirdir Lake during the summer and autumn of 2006. The blooms were seen as a bluegreen jelly or hand in shores of the lake. M. aeruginosa and M. flos-aqua were identified in our investigation but M. aeruginosa was determined as the bloorn-forming species. M, aeruginosa was counted as 16,530 colony/L and Chlorophyll-a concentration was measured as 117 mu g L-1 in Our investigation. In this respect, we can say that Egirdir Lake is under the potential risk of cyanobacterial toxicity. Surface water temperature was recorded as 24.2 degrees C. Orthophosphate-P (PO4-P) and inorganic nitrogen (NO2-N, NO3-N and NH4-N) concentrations were not detected from the sampling station. Details of the other environmental parameters are given in the text.