Background information. Rho GTPases regulate a wide range of cellular functions affecting both cell proliferation and cytoskeletal dynamics. They cycle between inactive GDP‐ and active GTP‐bound states. This cycle is tightly regulated by GEFs (guanine nucleotide‐exchange factors) and GAPs (GTPase‐activating proteins). Mouse CdGAP (mCdc42 GTPase‐activating protein) has been previously identified and characterized as a specific GAP for Rac1 and Cdc42, but not for RhoA. It consists of an N‐terminal RhoGAP domain and a C‐terminal proline‐rich region. In addition, CdGAP‐related genes are present in both vertebrates and invertebrates. We have recently reported that two predominant isoforms of CdGAP (250 and 90 kDa) exist in specific mouse tissues.Results. In the present study, we have identified and characterized human CdGAP (KIAA1204) which shares 76% sequence identity to the long isoform of mCdGAP (mCdGAP‐l). Similar to mCdGAP, it is active in vitro and in vivo on both Cdc42 and Rac1, but not RhoA, and is phosphorylated in vivo on serine and threonine residues. In contrast with mCdGAP‐l, human CdGAP interacts with ERK1/2 (extracellular‐signal‐regulated kinase 1/2) through a region that does not involve a DEF (docking site for ERK Phe‐Xaa‐Phe‐Pro) domain. Also, the tissue distribution of CdGAP proteins appears to be different between human and mouse species. Interestingly, we found that CdGAP proteins cause membrane blebbing in COS‐7 cells.Conclusions. Our results suggest that CdGAP properties are well conserved between human and mouse species, and that CdGAP may play an unexpected role in apoptosis.