In killer strains of the yeast Kluyveromyces lactis, production of a protein toxin which inhibits the growth of sensitive yeast cells is associated with the presence of two linear DNA plasmids, k1 and k2. We have determined the nucleotide sequence of the smaller plasmid k1 (8.9kb) which is thought to carry the structural gene(s) encoding the toxin. The plasmid has a low G + C content (26.8%) and contains four long open reading frames which account for over 95% of the total sequence. The longest open reading frame (1146 amino acids) probably corresponds to a structural gene for the killer toxin. Transcripts from three of the putative genes have been detected in K.lactis by Northern hybridisation.