Rice blast disease is one of the important diseases in Egypt as well as in most of the rice growing countries. This study was conducted to clarify the host range of this disease in Egypt. Isolation of the causal organism of rice blast disease Pyricularia grisea was obtained from some rice commercial cultivars i.e. Giza 171, Giza 176, Reiho and the susceptible check variety Giza 159, from both Sakha and Gemmiza experimental farms. Also, some isolates were obtained from different weeds collected from rice growing governorates during 1999 and 2000 seasons. Eight rice isolates were identified as 6 races on the international differential varieties, while fifteen weed isolates were identified as 4 races. Some weeds were found to serve as secondary hosts for rice blast fungus, i.e. Cyperus rotundus L., Cyperus alopecuroides R. Ottb., Echinochloa crus-galli L. Beauv., Echinochloa colona L. Link., Elusine indica L. Gaern., Dinebra retroflexa Panz., Panicum repens L., Leersia hexandra Sw., Digitaria nodosa Parl., Setaria verticillata L. Beauv. and Paspalum distichum L. The identified isolates of rice race groups were IB (37.5%), IC (25%), IG (25%) and ID (12.5%), while the identified weed races were IG-1 (6.6%), IB (26.7%) and ID (6.7%). All rice physiological races were able to infect the susceptible rice cultivars, as well as some new resistant cultivars as; Sakha 104, Sakha 101, Giza 177, Giza 181 and Giza 182 and some weeds during 1999 and 2000. While some weed races were able to infect their original hosts and some old susceptible rice cultivars. Also, some weed races were able to infect other weeds besides their original hosts. Some new virulent and specific blast races to rice cultivars Sakha 104 and Sakha 101 were appeared and caused a breakdown of resistance genes of both cultivars. Some races were virulent to Sakha 104 cultivar only, and the other specific for infecting Sakha 101. While some races able to infect both cultivars and the other were avirulent to the same cultivars during the period from 1999 to 2008.