Some scleractinian corals are susceptible to an infection called black band disease caused by the cyanobacterium Phormidium corallyticum Rutzler & Santavy. A similar disease, also caused by P. corallyticum, was discovered on colonies of the gorgonians Pseudopterogorgia acerosa (Pallas) and Pseudopterogorgia americana (Gmelin). In these gorgonians, the infection progressively denudes the skeleton of coenenchymal tissues as it moves along the main axis and peripherally onto branchlets. Subsequent colonization by macrophytic algae and epibiotic invertebrates smothers the remnants of living gorgonian tissue leading to further damage and possible death. Diseased colonies were found in 0.5 to 2.5m water depth in the northern Florida Keys. Highest disease frequencies (8% of all colonies) were found on P. acerosa during summer in the shallowest depth zone at Sands Key, Florida. The population of P. acerosa within a permanent quadrat slowly declined over two years at a rate slightly higher than disease frequency.