Cytological and genetical studies were carried out on the possible mechanism(s) involved in the production of new physiologic races of the yellow rust pathogen, Puccinia striiformis. The size and structure of dividing and non-dividing nuclei in hyphae and basidiospores was ascertained using light and electron microscopy. The parallel observations of the dividing somatic nucleus by the two techniques permitted a relatively complete sequence of events to be established for mitosis. Chromosome studies revealed that P. striiformis has a basic chromosome number of three, although some nuclei from basidiospores were seen to possess four chromosomes. Preparations in which hyphae were possibly fusing were observed using the light and electron microscope. The results of the cytological work are discussed with reference to the physical basis of genetic variation in the yellow rust fungus. Genetical studies included tests on the progeny of a mixture of two races, following the inoculation of susceptible and differentially resistant varieties. A new race, designated 105E137> was identified following the mixed inoculation of races 104E137 and 33E32. Induced mutation experiments established that the rate of mutation in the pathogen for specific virulence factors was less than one in 2 x 10 . The results of the study are discussed and genetic models are presented to explain how race 105E137 may have evolved. In a series of experiments on quantitative variation in P^_ striiformis differences between isolates in spore production and colony growth rate were established. An isolate of race *fOE8 was the most prolific genotype in experimental studies on a range of host varieties. Isolates tended to rank similarly on the test varieties although there was some evidence of isolate-host interactions. Detailed studies on the various stages of infection revealed that the prolificacy of the race 40E8 isolate could be detected at all stages of pathogen development.