We have determined the nucleotide sequence of group II RNA phage KU1. The most conspicuous difference in the comparison with other group II members such as GA and JP34 is the presence of an insertion in the start codon of the lysis gene. In GA and JP34, the coat and lysis genes overlap by one nucleotide in the configuration UAAUG. The 18-nt insertion in KU1 is positioned between the A and the U of the start codon. It does not affect the coat reading frame, but it destroys the AUG start codon and separates the previously overlapping genes by 17 nts. The insert creates a UUG codon at its 3' border which serves as the start site for lysis protein synthesis in KU1. We also show that analogous to the group I phages, such as MS2 and fr, expression of the lysis gene in KU1 and JP34 is coupled to termination of translation at the coat gene. RNA secondary structure models for the central parts of KU1 and JP34 are suggested which can account for the insertion as a separate stem-loop structure.