The large subunit of the human pre-messenger RNA splicing factor U2 small nuclear ribonucleoprotein auxiliary factor (hU2AF 65 ) is required for spliceosome assembly in vitro. A complementary DNA clone encoding the large subunit of Drosophila U2AF (dU2AF 50 ) has been isolated. The dU2AF 50 protein is closely related to its mammalian counterpart and contains three carboxyl-terminal ribonucleoprotein consensus sequence RNA binding domains and an amino-terminal arginine- and serine-rich (R/S) domain. Recombinant dU2AF 50 protein complements mammalian splicing extracts depleted of U2AF activity. Germline transformation of Drosophila with the dU2AF 50 complementary DNA rescues a lethal mutation, establishing that the dU2AF 50 gene is essential for viability. R/S domains have been found in numerous metazoan splicing factors, but their function is unknown. The mutation in Drosophila U2AF will allow in vivo analysis of a conserved R/S domain-containing general splicing factor.