The most surprising result emerging from our analysis of MinJ was that the arrest of cell division in minJ mutants occurs post FtsZ ring formation. We then showed that DivIVA mutants also arrest at this later step. Our previous report that divIVA mutants are affected at the earlier step of FtsZ assembly was based on immunofluorescence microscopy (Marston et al., 1998). Immunofluorescence experiments require damaging fixation and permeabilization treatments, which may have resulted in reduced visibility of FtsZ structures, or perhaps the rings formed in divIVA mutants are more fragile. In any case, a block in Z ring assembly was also anticipated based on numerous studies of the Min systems of both B. subtilis and E. coli. There is one report of a block in recruitment of FtsA to the Z ring in E. coli (Justice et al., 2000) but all other studies, both in vivo and in vitro, have supported an effect on Z ring assembly (Bi and Lutkenhaus, 1990; de Boer et al., 1990; Hu and Lutkenhaus, 1999; Levin et al., 2001; Pichoff and Lutkenhaus, 2001; Shiomi and Margolin, 2007; Dajkovic et al., 2008). In all of the in vivo studies the effect of MinCD was assessed in cells overproducing MinC or a tagged derivative. These experiments and the in vitro data demonstrate overwhelmingly that MinC can act on Z assembly. Furthermore, recent results show that E. coli MinC, at least, has two different activities affecting FtsZ assembly dynamics (Dajkovic et al., 2008). However, in the light of our new data, we suggest that, at least in B. subtilis, and when expressed at normal physiological levels, the MinCD inhibitor mainly acts downstream of FtsZ ring assembly, preventing the recruitment of the late group of transmembrane division proteins. In support of this idea we note that there are at least two other situations in which the natively expressed MinCD system of B. subtilis seems not to inhibit Z ring assembly. The first is in ezrA mutants, which make extra Z rings close to the cell poles, despite the presence and activity of MinCD (Levin et al., 1999). Again, these rings do not mature into active division sites presumably because of a downstream action of MinCD. Second, during sporulation, in which polar septa are formed, again, despite the continued presence of the MinCD inhibitor in its normal polar zones (Barak et al., 1998; Sharp and Pogliano, 2002). More work is needed to determine whether physiological concentrations of MinCD act by specifically blocking the recruitment of one or more late division proteins, or whether they simply reduce some feature of FtsZ assembly to the point where the late proteins cannot efficiently be recruited. Nevertheless, it is clear that several proteins, including FtsA, EzrA, SepF and ZapA, do associate with FtsZ and that specifically positioned ring- or helix-like structures can be formed.