Proteins that associate with the GTP‐bound forms of the Ras superfamily of proteins are potential effector targets for these molecular switches. A 195 kDa protein was purified from cell lysates by affinity chromatography on immobilized cdc42Hs‐GTP and a corresponding cDNA was isolated. Sequence analysis revealed localized identities to calponin, the WW domain, unconventional myosins and to the rasGAP‐related domain (GRD) contained in IRA, NF‐1, SAR1 and rasGAP. p195 was found to be identical to IQGAP1, a protein previously reported to bind ras. Purified recombinant p195/IQGAP1 bound to and inhibited the GTPase activity of cdc42Hs and rac whereas no interaction with ras was detected. The C‐terminal half of IQGAP1 containing the GRD bound to cdc42 and rac in a GRD‐dependent fashion, but a smaller fragment containing only the GRD did not. Cdc42 was also co‐immunoprecipitated from cell lysates with antibody specific to p195/IQGAP1. Calmodulin also co‐immunoprecipitated with p195/IQGAP1 and was found to associate with fragments containing the IQ domain. Expression of a cDNA fragment encoding the GRD inhibited the CDC24/CDC42 pathway in yeast, but no effect on ras was observed. In mammalian cells, both endogenous and ectopically expressed p195/IQGAP1 were localized to lamellipodia and ruffling cell membranes, where co‐localization with actin was apparent. These results suggest that IQGAP1 is an effector target for cdc42Hs and may mediate the effects of this GTPase on cell morphology.