The Bacillus subtilis cell-division protein DivIB is shown to be present at an approximately 100-fold higher abundance (approximately 5000 molecules per cell) than its Escherichia coli FtsQ homologue. B. subtilis contains much more DivIB (at least 60-fold) than is needed to maintain the normal rate of cell division at moderate temperatures (up to 37 degrees C). However, a high level of DivIB is needed to achieve the normal rate of division at high temperature (47 degrees C). It is proposed that membrane-bound DivIB is involved in stabilizing or promoting the assembly of the division complex (which is intrinsically temperature sensitive) in a manner that requires more of the protein at higher temperatures. The (at least) 60-fold accumulation of DivIB and FtsZ from an undetectable level, following germination and outgrowth of spores up until the stage of the first cell division, was unaffected by blocking of initiation of the first round of replication. It is concluded that there is no major synthesis of either of these 'division initiation' proteins linked to initiation, progression or completion of the first round of replication accompanying spore outgrowth.