The production of leuconocin S, a bacteriocin from Leuconostoc paramesenteroides OX, was examined during batch fermentations at pH 6.0, 6.5, 7.0 and 7.5. The specific growth rate and bacteriocin titers were highest at pH 7.0, with values of 0.33 h(-1) and 20 000 AU/ml, respectively. Maximum bacteriocin production corresponded to late logarithmic-early stationary growth phase. Bacteriocin levels decreased during late stationary phase, possibly due to proteolytic enzymes produced during this period.