Using a procedure that minimizes shear forces, the BamHI-derived forked termination of replication intermediate of Bacillus subtilis, called band I DNA, can be extracted with little or no accompanying band II DNA. It has been shown that band II DNA is a product of band I breakdown. Nuclease P1-mediated breakdown of the forked band I DNA proceeds in two steps. The first causes the release of one of the arms as band II DNA; in the second step, the remaining arm is cleaved away to yield the free stem. It is concluded that band I represents the primary termination of replication intermediate. A quantitative assessment of the level of band I in DNA from cells of the merodiploid strain, GSY1127, growing at different rates has been made. For cells grown in a minimal medium, at least, the experimentally measured level of band I is of the order (approx. 60%) of that predicted for a complete block to movement of the clockwise fork at the replication terminus, erC.