The two major pathotypes (W & R) of the cereal eyespot fungus, Pseudocercosporella herpotrichoides, show variation of major agronomic importance, specifically in sensitivity to fungicides and in differences in host species specificity. This thesis describes the genetic analysis of these characters using the parasexual cycle to generate recombinant progeny between W and R pathotypes. Interspecific hybrid progeny from a parasexual cross between P. herpotrichoides and P. anguioides were included in the analysis. Marker differences present in the parental isolates including auxotrophic requirements, isozyme banding patterns, sensitivity towards benzimidazole fungicides and spore length have been used as proof of recombination in the parasexual progeny, and to characterise the pattern of genetic segregation in the crosses. Analysis of fungicide sensitivity focussed on the ergosterol biosynthesis inhibitors (EBI), where it is found that W-types are significantly more sensitive to the majority of the déméthylation inhibiting (DMI) fungicides than are the R-tvpes. Determination of sensitivity of progeny to EBI fungicides revealed the presence of 'major' and 'minor' gene(s) controlling the expression of fungicide resistance. Major gene segregation resulting in a substantial difference in sensitivity to triadimenol, was found to occur prior to exposure of the diploid fusion products to haploidizing agents with the result that progeny generated from any one fusion product were either sensitive or resistant to triadimenol. Segregation of additional minor genes occurred following exposure to haploidizing agents and these modified the level of resistance expression within the progeny. These analyses provided the first genetic evidence for cross-resistance to these fungicides in this fungus. Cross-resistance relationships were shown to depend on the mode of action of the chemicals and also varied within the chemical groups. Interaction was seen between major and minor gene(s) in the expression of cross-resistance relationships. Pathogenicity towards wheat and rye was shown to be considerably disrupted by parasexual recombination. Although most paraexual progeny were not pathogenic a minority of isolates were able to infect and produce typical disease symptoms on wheat (14%) and / or rye (21%). Pathogenicity to wheat and rye was not associated indicating that the inheritance of pathogenic ability towards wheat and rye is genetically separable and that new 'pathotvpes' can be generated through recombination. Withm the non-pathogenic progeny a subset of recombinant strains were found which were able to colonise host tissues demonstrating a clear parasitic ability. The remaining progeny appeared to be deficient in both pathogenic and parasitic abilities. These three groups of recombinants : pathogenic, parasitic/non-pathogemc and nonparasitic will allow the future investigation of both pathogenicity and parasitism in this fungus. Parasexual progeny were used to investigate the inheritance of vegetative incompatibility in P. herpotrichoides. However, detection of vegetative incompatibility between W - and R-tvpe isolates or between recombinant progeny was not successfull in spite of using a number of different techniques. Further refinement of the assay system is required.