Antigenic sites in the cucumber mosaic virus (CMV) coat protein (CP) have been identified using a polyclonal antiserum prepared against glutaraldehyde-fixed virions. Antibodies were used to screen a random peptide library of heptamers displayed on the surface of a bacteriophage. Eight of 36 (22%) sequenced phage clones had inserts resembling a putative virion surface domain of the CMV CP. This region has the sequence LETDEL, corresponding to amino acids 194-199 in the Fny-CMV CP. The binding of phage clones to Fny-CMV antiserum was inhibited by a synthetic peptide representing this region. Six of 36 (17%) phage clones contained sequences corresponding to a C-terminal sequence in the Fny-CMV CP, which is thought to be internal in assembled virions. This sequence, EHQRIPTSGV, represents amino acids 206-215 and all but the P residue were observed in at least one clone. Four of 36 (11%) sequenced phage clones carried sequences that matched a portion of the sequence RLLLPDSV, corresponding to amino acids 89-96 in the Fny-CMV CP. This region was also identified as the antigenic site recognized by a monoclonal antibody (MAb23C10E4). Eleven percent of the phage (4 of 36) contained sequences matching at least three amino acids of the N-terminal region in the CMV CP. The positions of the antigenic sites seen in this study are consistent with a predicted structure for the CMV CP.