Four bacterial isolates have been obtained which cause lysis of laboratory cultures and natural populations of blue-green algae. Over forty algal strains belonging to all orders of the blue-green algae and including bloom-forming species are susceptible. The bacteria also lyse certain gram-positive and gram-negative bacteria. The organisms are aerobic, gram-negative rods, aflagellate, motile and pigmented with an absorption maximum of the pigments in methanol occurring at 435 nm and with shoulders at 460 nm and 415 nm. The guanine and cytosine in the DNA is within the range of 65–69± 1 M%. The organisms are identified tentatively as members of the Myxobacterales. Lysis of vegetative cells of the algae depends on the presence of growing bacteria, bacteria-free filtrates alone being insufficient to cause lysis. Heterocysts are not lysed. The bacterial plaques resemble viral plaques in appearance. The lytic agents rapidly inhibit metabolic activity of the susceptible algae and may cause cell lysis within 2–10 hours. The isolates can be propagated readily in the absence of the host and are probably widely distributed in freshwater habitats.