The influence of dilution rate (D) and aeration on soluble and cell-bound nisin Z production was investigated during continuous free (FC) and immobilized cell (IC) cultures with Lactococcus lactis subsp. lactis biovar diacetylactis UL719 in supplemented whey permeate. Maximum total bacteriocin titres during non-aerated continuous FC and IC cultures were obtained for low D, with 1490 and 1090IUmL - 1 for 0.15h - 1 or 0.25 and 0.5h - 1 , respectively. For both systems, aeration increased nisin total production with maximum titres of 2560 and 2430IUmL - 1 for low D, respectively, as well as specific production. Volumetric productivity was the highest for an intermediate D of 0.4h - 1 during FC cultures (460IUmL - 1 h - 1 for both aerated and non-aerated cultures), while it increased continuously with D during IC cultures, reaching high values of 1090 and 1760IUmL - 1 h - 1 at 2.0h - 1 without and with aeration, respectively. In comparison with previous data for FC batch cultures, data from this study may indicate that during continuous fermentations at steady state, some steps in nisin biosynthesis are limiting. In these conditions, nisin production by immobilized cells is reduced.