ABSTRACTThe relatedness between fourPhytophthora megaspermaisolates [P. megaspermaf. sp.glycinea(Pmg) from soybean,P. megaspermaf. sp.medicaginis(Pmm) from alfalfa, and two broad host-range isolates from apple or alfalfa],P. cactorum, P. cryptogea, andP. parasiticavar.nicotianae(Ppn) was investigated by a quantitative analysis of restriction fragment length polymorphisms of mitochondrial DNAs. Nucleotide sequence divergence values amongP. megaspermaisolates were within the same range as between theP. megaspermaisolates and the three otherPhytophthoraspecies. An unrooted phylogenetic network based on nucleotide sequence divergence values indicated that several evolutionary lines exist forP. megasperma.These data are consistent with the observed variability of morphological, physiological and biochemical characteristics within this fungal group and support the contention thatP. megaspermacomprises several biological species which are the result of convergent evolution and which are not closely related. A consensus restriction map of the mtDNAs of Pmg, Pmm, and Ppn was constructed, and numerical analyses of conserved and variable restriction cleavage sites also support the separation of Pmg and Pmm into distinct species.