The Eph receptor tyrosine kinases are overexpressed in many pathologic tissues and have therefore emerged as promising drug target candidates. However, there are few molecules available that can selectively bind to a single Eph receptor and not other members of this large receptor family. Here we report the identification by phage display of peptides that bind selectively to different receptors of the EphB class, including EphB1, EphB2, and EphB4. Peptides with the same EphB receptor specificity compete with each other for binding, suggesting that they have partially overlapping binding sites. In addition, several of the peptides contain amino acid motifs found in the G-H loop of the ephrin-B ligands, which is the region that mediates high-affinity interaction with the EphB receptors. Consistent with targeting the ephrin-binding site, the higher affinity peptides antagonize ephrin binding to the EphB receptors. We also designed an optimized EphB4-binding peptide with affinity comparable with that of the natural ligand, ephrin-B2. These peptides should be useful as selective inhibitors of the pathological activities of EphB receptors and as targeting agents for imaging probes and therapeutic drugs.