We have cloned the early region of the major leftward operon of bacteriophage phi 80 DNA and determined the nucleotide sequence of the PL-nutL-tL1 segment. The genes and sites equivalent to lambda gene N, antiterminator nutL, and terminator tL1 were found in this region by functional tests using the recombinant plasmids carrying the transcriptional units Ptrp-tL1-galK, Ptrp-nutL-tL1-galK and PlacUV5-N. Sequence homologies were observed between the boxA of the phage 21 nutR site and the inverted repeat of the nut core in the nutL-equivalent site of phi 80. The N-equivalent gene of phi 80 codes for a protein with 98 amino acids. It is highly basic as is the lambda N protein. Recombinant plasmids carrying the phi 80-PL promoter and nut-equivalent site of phi 80 are lethal to the Escherichia coli host cell, but can be stably maintained in the presence of the phi 80-cI repressor.