Abstract Rho GTPases regulate a number of important cellular functions in eukaryotes, such as organization of the cytoskeleton, stress-induced signal transduction, cell death, cell growth, and differentiation. We have conducted an extensive screening, characterization, and analysis of genes belonging to the Ras superfamily of GTPases in land plants (embryophyta) and found that the Rho family is composed mainly of proteins with homology to RAC-like proteins in terrestrial plants. Here we present the genomic and cDNA sequences of the RAC gene family from the plant Arabidopsis thaliana. On the basis of amino acid alignments and genomic structure comparison of the corresponding genes, the 11 encoded AtRAC proteins can be divided into two distinct groups of which one group apparently has evolved only in vascular plants. Our phylogenetic analysis suggests that the plant RAC genes underwent a rapid evolution and diversification prior to the emergence of the embryophyta, creating a group that is distinct from rac/cdc42 genes in other eukaryotes. In embryophyta, RAC genes have later undergone an expansion through numerous large gene duplications. Five of these RAC duplications in Arabidopsis thaliana are reported here. We also present an hypothesis suggesting that the characteristic RAC proteins in higher plants have evolved to compensate the loss of RAS proteins.