The Entamoeba histolytica small GTP-binding protein EhRho1 has an unusual amino acid residue at a conserved site found in all known Ras superfamily proteins. EhRho1 has an isoleucine at position 45, which corresponds to position 28 of human Ras and Rac and position 30 of human Rho and Cdc42. All other known small GTPases have an aromatic residue (typically phenylalanine) at this position, and mutation to a leucine renders other Ras proteins constitutively active by reason of diminished affinity for GDP. It was determined that the EhRho1 protein has a half-time of GDP dissociation similar to that of a human Rho protein, HsRhoA, and therefore an isoleucine at this site in EhRho1 is not likely to render EhRho1 constitutively active. It was also found that EhRho1 is not a substrate for the Rho-specific C3 exoenzyme. Thus EhRho1 appears to be an unusual member of the Ras family.