The energy conservation and number of viable cells of Nitrosomonas europaea fluctuate dramatically during cultivation. In discontinuous Culture the specific activity (SA) reaches its maximum after 9 h with about 2700 nmol O(2) (mg protein)(-1) min(-1), where the highest number of viable N. europaea cells is detectable after 21 h with 2 x 10(8) cell ml(-1). Afterwards, both SA and viable cell number immediately start to decrease. Accordingly, the exponential growth turns into a linear growth, whereby the number of viable cells permanently decreases. The exponential growth phase can be extended from about 21 to 38 h by increasing tire concentration of CO(2) or trace elements. In continuous fermentation of N. europaea, SA of about 2500 nmol O(2) (mg protein)(-1) min(-1) and viable cell number of 2.5 x 10(8) cell ml(-1) is detectable at dilution rates between 1 and 1.8 day(-1). At dilution rates below 1 day(-1), SA and number of viable cells are reduced. The minimal doubling time is 13 and 15 h during continuous and discontinuous fermentation respectively. Consequently, cell production of N. europaea should be performed in Continuous fermentation. When bacteria are grown in discontinuous systems, they should be harvested in the early exponential growth phase. (C) 2009 Elsevier Ltd. All rights reserved.