The effect of Phytophthora infestons culture filtrate (CF) upon the ultrastructure of cellular organelles in potato tuber and stem derived calli has been studied. During the callus growth at light a transformation of proplastides in chloroplasts has been observed: first, continuous thylakoid membranes are formed around the starch inclusion, secondly, the assembly in grana of thylakoid took place. In CF treated calli the following changes were noted: (a) transformation of the cellular content into amorphous mass indicating the necrosis of the tissues; (b) distrophy of the nucleus; (c) disintegration of the plastid and peroxisome envelopes and of the cytoplasmic membrane, which resulted in vesicles and (d) swelling of the mitochondria followed by fragmentations of cristae and matrix lysis. CF resistant calli having normal growth presented more developed photosynthetic apparatus in stem derived calli.