Rap1 is a Ras-related GTPase that is principally involved in integrin-and E-cadherin-mediated adhesion. Rap1 is transiently activated in response to many incoming signals via a large family of guanine nucleotide exchange factors (GEFs). The lack of potent Rap1 dominant-negative mutants has limited our ability to decipher Rap1-dependent pathways; we have therefore developed a procedure to generate such mutants consisting in the oligonucleotide-mediated mutagenesis of residues 14–19, selection of mutants presenting an enhanced interaction with Epac2 by yeast two-hybrid screening and counter-screening for mutants still interacting with Rap effectors. In detail analysis of their interaction capacity with various Rap-GEFs in the yeast two-hybrid system revealed that mutants of residues 15 and 16 interacted with Epacs, C3G and CalDAG-GEFI, whereas mutants of position 17 had selectively lost their ability to bind CalDAG-GEFI as well as, for some, C3G. In cellular models where Rap1 is activated via endogenous GEFs, the Rap1 [S17A] mutant inhibits both the cAMP–Epac and EGF–C3G pathways, whereas Rap1 [G15D] selectively interferes with the latter. Finally, Rap1 [S17A] is able to act as a bona fide dominant-negative mutant in vivo since it phenocopies the eye-reducing and lethal effects of D-Rap1 deficiency in Drosophila, effects that are overcome by the overexpression of D-Epac or D-Rap1.