Toxicity tests were designed to find out safe level of toxicants and different lethal concentration values for a particular organism or for different types of organisms. In the present investigation, a graded series of concentra- tions of the cadmium chloride was prepared ranging from 0.1 to 1.0 mg.l-1 (micro-range), to evaluate the toxic effects of cadmium chloride on the blue-green alga and to find out maximum allowable concentration of the toxicant. With the increase in the concentration of the toxicant the percent survival decreased and hundred percent deaths were noticed at 0.94 mg.l-1 of cadmium chloride with in a period of 15th days. With the increase in exposure period the concentration of cadmium decreased at a particular lethal dose/concentration value. The maximum allowable concentration (MAC) recorded for this alga for 15th days exposure was 0.21 mg l-1. The lethal concentration values for 15th days of exposure pe- riods have been outlined below. The LC10 value was 0.34 mg l-1. The LC50 value was 0.58 mg l-1. LC90 value was 0.84 mg l-1  and LC100 value was 0.94 mg.l-1, for this particular alga, Anabaena cylindrical. The control set showed 100% survival 100 through out the experimental periods. The dry weight showed an initial increase at conc. A up to 6th day of exposure and with the increase in exposure period the dry weight decreased significantly. At conc. B and C, the dry weight decreased significantly at all exposure periods. Complete bleaching of the algal mass inside the test solution (B & C) was observed from 3rd day of exposure period onwards. Gradually tiny blue-green particles started making their appearance in the white turbid mass as observed in the naked eye after 5th days of recovery period onwards.