The product of the spoIIM gene of Bacillus subtilis is required for complete septum migration and forespore engulfment during sporulation. To investigate whether expression of spoIIM is required in the forespore compartment of the sporangium, we have constructed a new integrational vector, pKSV7, which contains temperature-sensitive replication functions derived from pE194ts. The presence of the conditionally defective replication origin greatly stimulates plasmid excision when sporulation occurs at the permissive temperature. This facilitates the use of a genetic technique employed by Illing et al to distinguish genes whose expression must occur in the forespore from genes that may be expressed exclusively in the mother cell compartment. The results of the integration/excision experiments using pKSV7 support the conclusion that spoIIM must be expressed in the forespore. Biochemical analysis of forespore and mother cell fractions suggests that spoIIM is also expressed in the mother cell. The conditional integrational vector pKSV7 replicates at high copy number in E coli and allows the identification of inserts in the polylinker cluster by disruption of alpha-complementation and thus should be useful for other kinds of genetic manipulations in B subtilis.