SUMMARY.A disease of cultivated mushrooms is described in which patches of a chestnut brown colour so disfigure the pileus as to render the affected crop quite unmarketable.The disease is identical with that described in America by Tolaas, but left unnamed by him.The cause of the disease is a small bacterial parasite which may possibly be a strain ofPseudomonas fluorescensand may prove to be identical with the organism which produces Pear‐blossom Blight.Until its identity is established by further experiment it seems well to give it a distinguishing mark, and the namePseudomonas Tolaasiis suggeste