The ascomycete fungus Magnaporthe oryzae, causative agent of rice blast disease, poses a threat to global food security, destroying enough rice to feed 60 million people each year. Characterization of the host-pathogen interaction between rice and M. oryzae is critical, as better understanding of the system may lead to better disease control strategies. The sequenced genome and repertoire of molecular tools available have made M. oryzae an ideal model system for understanding general plant-pathogen interactions as well. The objective of this dissertation was to characterize the M. oryzae homologs of Histoplasma capsulatum RYP (Required for Yeast Phase) genes that are required for transition to the parasitic phase. H. capsulatum is a human pathogen that undergoes a dimorphic switch from filamentous to yeast cell growth at 37°C, the host body temperature. Four H. capsulatum RYP genes were identified in a forward genetic screen to identify genes required for entry into the yeast phase. RYP1 is a member of the Gti1_Pac2 family, which contains previously characterized regulators of dimorphic switching. RYP2 and RYP3 are homologs of vosA and velB, members of the Velvet family, best characterized in Aspergillus nidulans, where they coordinate morphological differentiation with secondary metabolism. RYP4 is a zinc binuclear cluster protein, a main class in the zinc finger transcription factor family. Deletion of the M. oryzae RYP1 homolog, RIG1 (Required for Infectious Growth), resulted in a non-pathogenic mutant on susceptible rice cultivars, even upon removal of the host penetration barrier. Δrig1 was blocked in the transition to infectious hyphal growth, similar to H. capsulatum ryp1, which could not transition to the yeast phase. Deletion mutants of M. oryzae RYP2, RYP3, and RYP4 homologs were similar to the wild type in somatic growth and pathogenicity indicating that although RIG1 is a pathogenicity factor conserved in plant and animal pathogens, such conservation does not apply to all of the RYP pathogenicity genes identified in H. capsulatum. Δrig1 is the first M. oryzae mutant known to be blocked in production of primary infection hyphae. Overall, the study suggests limited parallels exist in phase transition of fungal pathogens of plants and animals.