The development of Polyporus brumalis basidomycete cultures, both in the dark and in the light, was studied in the 17 day experiment on the orbital station Salyut 5 and in the 20 day experiment on the orbital station Salyut 6. In the first experiment the test tube cultures were exposed to weightlessness beginning from the stage of the fruiting body primordium. In the second experiment the culture was exposed in large containers at the stage of mycelium growth. In the first experiment fruiting bodies that had formed in light had stems and caps approximating the appearance and anatomical structure of the control culture. The fruiting bodies and caps were oriented towards the light. Some structural changes in the hymenophore were observed. Fruiting bodies developed in the dark had twisted stems and no caps. In the second experiment no fruiting bodies were formed in the dark. Light-optical and electron-microscopic studies of Polyporus brumalis growth under weightlessness were carried out.