For the first time a mycoplasma has been isolated from fish. The organism, designated strain 163K, was isolated on modified Hayflick medium under aerobic conditions at 25 degrees C from the gills of a tench (Tinca tinca L.). It showed the characteristic features of mycoplasmas. In addition it was flask-shaped with a distinct head-like structure and was able to attach to inert surfaces and living cells. The most interesting property of the organism was its ability to show fast gliding motion. Movement was only in the direction of the head-like structure and was not interrupted by resting periods.