The impact of some organotin compounds (10-200 mg dm^3) on the planktonic cyanobacterium Synechocystis aquatilis was studied. The following order of toxicity of organotins tested to the cyanobacterium culture was found: DBTCl>TPTAclTBTCl? TPTCl>TMTCl. Chlorophyll a content in the culture seemed to be more susceptible to organotins than cyanobacterial growth. The inhibition of both parameters increased significantly with the increase of compound doses, time of exposure and decrease of initial culture density. After 96 h exposure, the chlorophyll a content in the cyanobacte-rium cultures of the initial density 56 mg dry wt dm^3 was reduced for 200 mg dm^3 of DBTCl, TBTCl and TPTCl by 70%, 50% and 20% respectively, while in the cultures of the lowest initial density (10 mg dry wt dm-3) by 90%, 75% and 50%, as compared to the organotin free controls.