In a membrane bioreactor (MBR), fast growth of anammox bacteria was achieved with a sludge residence time (SRT) of 12 days. This relatively short SRT resulted in a—for anammox bacteria—unprecedented purity of the enrichment of 97.6%. The absence of a selective pressure for settling, and dedicated cultivation conditions led to growth in suspension as free cells and the complete absence of flocs or granules. Fast growth, low levels of calcium and magnesium, and possibly the presence of yeast extract and a low shear stress are critical for the obtainment of a completely suspended culture consisting of free anammox cells. During cultivation, a population shift was observed from Candidatus “Brocadia” to Candidatus “Kuenenia stuttgartiensis.” It is hypothesized that the reason for this shift is the higher affinity for nitrite of “Kuenenia.” The production of anammox bacteria in suspension with high purity and productivity makes the MBR a promising tool for the cultivation and study of anammox bacteria. Biotechnol. Bioeng. 2008;101: 286–294. © 2008 Wiley Periodicals, Inc.