Phymatotrichum Omnivorum (Shear) Duggar is a soiI borne fungus that causes cotton root rot and diseases of a great variety of crops in the Southwestern states. Little information is available regarding the variation of the fungus. The objective of this research is to define the genetic variation of P. omnivorum at the DNA level by analyzing restriction fragment length polymorphisms (RFLPS) of a collection of 21 isolates isolated from various geographic origins and hosts. Four isolates, the Dallas, Pearsall and Temple isolates of cotton and one maple isolate were used in the initial tests. Total DNA was extracted from mycelium and purified by isopycnic centrifugation in cesium chloride/bisbenzimide density gradients. RFLPs were detected among the four isolates in ethidium bromide - stained gels and by Southern hybridization to cloned nuclear ribosomal DNA (rDNA) of Neurospora crassa. RFLPs will be further studied by using cloned probes of other known genes or random fragments of total P. omnivorum DNA to determine the genetic relationship among the isolates. Extrachromosomal DNAs of unknown origin were observed in total DNA preparation of the four isolates. The presence of these extrachromosomal DNAs and their polymorphisms among all isolates will be investigated.