Specific targeting of the integrin subtype alpha 5 beta 1 possesses high potential in cancer diagnosis and therapy. Through sequential N-methylation, we successfully converted the biselective alpha 5 beta 1/alpha v beta 6 peptide c(phg-isoDGR-k) into a potent peptidic RGD binding aSfil subtype selective ligand c(phg-isoDGR-(NMe)k). Nuclear magnetic resonance spectroscopy and molecular modeling clarified the molecular basis of its improved selectivity profile. To demonstrate its potential in vivo, c(phg-isoDGR-(NMe)k) was trimerized with the chelator TRAP and used as a positron-emission tomography tracer for monitoring alpha 5 beta 1 integrin expression in a M21 mouse xenograft.