Mitochondrial DNA (mtDNA) of Phytophthora infestans has been isolated and preliminarily characterized. It has a low GC content of about 22.4% which is distinctly different from that of nuclear DNA (51 %). This property has been used to separate both DNA species in the presence of 4',6-diamidine-2-phenylindole (DAPI) in CsCl density gradients. The use of cetyl triammonium bromide (CTAB) for extraction of DNA significantly reduced its degradation. The base distribution of the mtDNA shows a limited intramolecular heterogeneity. The molecule contains 36.2 ± 0.3 kb as revealed by endonuclease digestion and seems to be circular as shown by restriction mapping. No differences were found in restriction patterns between mtDNAs from various pathotypes.