The inferred amino acid sequences of 10 specific gene products from nine retroviruses were aligned by computer, all evolutionary distances between them calculated, and evolutionary trees constructed. Not unexpectedly, the various gene products are changing at different rates, the reverse transcriptase being the least and the envelope proteins the most different from one retrovirus to another. For the most part, trees based on the retroviral enzyme sequences are congruent, indicating that extensive genetic recombination has not been a major factor in the evolution of the central part of the genome. In the case of envelope protein sequences, however, the sequences clearly exhibit evidence of multiple cross-over events between quite distantly related retroviruses. A composite phylogenetic tree was constructed from the four retroviral enzyme sequences, and a number of important historical happenings were interpreted in the light of the time scale it affords.