The TRP1 gene of the yeast Kluyveromyces lactis has been cloned from a genomic library by complementation of the Saccharomyces cerevisiae trp1-289 mutation. The gene was located within the clone by transposon mutagenesis and the coding region identified by DNA sequencing. This has indicated that K. lactis TRP1 encodes a 210-amino acid polypeptide which shows 53% identity to the homologous S. cerevisiae protein. The K. lactis TRP1 gene has been disrupted by substituting the S. cerevisiae URA3 gene for a large part of the TRP1 coding sequence. Replacement of the chromosomal TRP1 locus with this construction has enabled the production of non-reverting trp1- strains of K. lactis, while a genetic analysis of the disrupted allele confirmed that the TRP1 gene had been cloned. DNA sequencing has also shown that the K. lactis TRP1 sequence is flanked by genes encoding inorganic pyrophosphatase and histone H3, which we have designated IPP and HHT1 respectively. Hybridization studies have shown that in common with S. cerevisiae, K. lactis has two copies of the histone H3 gene. Each H3 gene is closely linked to a gene encoding histone H4 and in both yeast species the IPP gene is tightly linked to one of the histone gene pairs.