The DNA sequences of structural genes of several U.K. and European isolates of feline immunodeficiency virus (FIV) were determined and compared with those of other worldwide isolates. Phylogenetic analyses of both gag and env sequences demonstrate that a Japanese isolate represents a distinct sequence subgroup, with corrected amino acid distances to the other isolates averaging 23% in env and 8% in gag. Analysis also reveals that an evolutionary radiation of FIV occurred with many isolates diverging at approximately the same time, and that although isolates from similar geographical sources often cluster together, there is evidence of more than one origin for FIV in the U.K., The Netherlands and Italy. Estimation of the numbers of silent and replacement nucleotide substitutions indicates the presence of constraints against amino acid changes in gag and conserved regions of env but suggests that positive selection for protein sequence changes operates in variable regions of env. The possible immunological forces underlying these changes are discussed.