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The conversion from citrate positive (Cit+) to citrate negative (Cit-) phenotype of six strains of Leuconostoc mesenteroides was followed during growth in milk and buffered or unbuffered MRS medium at 30 or 37°C. High rate of loss of Cit+ phenotype was observed. The Cit- phenotype was found to be linked to the loss of 22 to 23 kb plasmids. All Cit- mutants isolated from Leuc. mesenteroides subsp. cremoris 195 reverted spontaneously to the Cit+ phenotype. Hybridization experiments using a 0.8 kb fragment of the citP gene of Leuc. mesenteroides showed that all the plasmids which were lost in Cit- mutants encoded for a citrate permease. However, neither plasmid nor genomic DNA from Leuc. mesenteroides subsp. cremoris 195 hybridized with the citP probe.