Comparative analyses of genome sequences from several plant-infecting fungi have shown conservation and expansion of protein families with plant disease-related functions. Here, we show that this hypothesis can be extended to mutualistic symbiotic fungi. We have identified a gene encoding an Era (Escherichia coli Ras)-like GTPase in the rice blast fungus Magnaporthe oryzae and found that it is orthologous to the mature amino terminal part of the Gin1 protein from the arbuscular mycorrhizal (AM) fungus Glomus intraradices. M. oryzae Erl1 is required for full root virulence. Appressoria formation was not severely affected in delta erl1 strains, but invasive hyphae grew slower than in the wild type. Root browning defect of delta erl1 strains could be complemented by the AM gene under the control of the ERL1 promoter. Erl1 and Gin-N localized to the nucleus when carboxy-terminally labeled with green fluorescent protein (GFP). However, amino-terminal GFP-tagged versions of the proteins expressed in Aspergillus nidulans were shown to localize in the cytoplasm and to cause polarity defects. These data suggest that Erl1 and Gin-N are orthologs and might be involved in the control of hyphal growth in planta. This is the first characterization of an Era-like GTPase in filamentous fungi.