The isolation and growth in sterile culture through sixteen passages, each of 4 to 6 weeks' duration, of an abnormal growth isolated from a gametophyte of Pteridium aquilinum var. latiusculum is reported, and the nutritional requirements described. The cultures consist of green, photosynthetic, predominantly filamentous, but coherent, masses. Prothalli were regenerated on five occasions during the first two passages only. Outgrowths of whitish or pale green parenchymatoua tissue occur. These possess a central strand of elongated cells, some of which become differentiated into a core of tracheides. Prothalli of the strain in culture are haploid. When examined soon after the initial explantation, the filamentous parts of the cultures were diploid, but there has been a subsequent increase in the number of chromosomes, and each cell is now irregularly aneuploid, with a chromosome number between triploid and tetraploid.