We have shown previously that, when spores of a thymine-requiring strain of Bacillus subtilis were grown out in the absence of thymine, mid-cell Z rings formed over the nucleoid and much earlier than might be expected with respect to progression into the round of replication. It is now shown that such conditions allow no replication of oriC. Rather than replication, partial degradation of the oriC region occurs, suggesting that the status of this region is connected with the 'premature' mid-cell Z ring assembly. A correlation was observed between entry into the replication elongation phase and a block to mid-cell Z rings. The conformation of the nucleoid under various conditions of DNA replication inhibition or limitation suggests that relief of nucleoid occlusion is not primarily responsible for mid-cell Z ring formation in the absence of thymine. We propose the existence of a specific structure at mid-cell that defines the Z ring nucleation site (NS). It is suggested that this NS is normally masked by the replisome upon initiation of replication or soon after entry into the elongation phase, and subsequently unmasked relatively late in the round. During spore outgrowth in the absence of thymine, this checkpoint control over mid-cell Z ring assembly breaks down prematurely.