The conjugative transposon Tn919 was introduced at high frequency to L. lactis subsp. lactis biovar. diacetylactis 18-16 and transconjugants were screened for mutations in two chromosomally located genotypes; citrate metabolism and maltose utilization. A citrate negative mutant, lacking citritase activity, was isolated at a frequency of 1.18 x 10(-4). The mutant, 18-16C5, contained a single copy of Tn919 in a chromosomal location. A junction fragment of Tn919::18-16C5 chromosomal DNA was cloned in Escherichia coli. Mutations in maltose metabolism were detected at a frequency of 4.0 x 10(-4). No mutants were detected when Tn919 was not introduced. Reversion to a Mal+ phenotype occurred at high frequency, but was not due to Tn919 transposition.