Cucumber mosaic virus (CMV) has an extremely broad plant-host range, a large number of vector species, and a wide geographical distribution. CMV is, therefore, a model by which to understand plant virus adaptation. The selective constraints exerted on the five proteins expressed from the CMV genome were evaluated by application of newly developed maximum-likelihood algorithms to analyze sequences available in data banks. The ratio between nonsynonymous and synonymous substitution rates (omega) was used to detect positive selection on particular codon sites. Amino acid sequences were conserved with omega ranging from 0.07 to 0.60 in different proteins. However, a small proportion of amino acids in proteins la, 2a, and 3b, the coat protein (CP), were positively selected (omega > 1). Moreover, the evolution of the CP in the three subgroups of CMV strains revealed different selection profiles along the sequence and significantly different speed of evolution at many positions. Constraints exerted by aphid transmission, rather than plant adaptation, seemed to be responsible for these patterns of evolution in the CP.