SCP1'-cysB carries a short chromosomal insertion: cysB was the only chromosomal locus identified on this plasmid; the plasmid was seldom lost; and a comparatively low frequency of crossing-over between the exo- and endogenote in a cysB+/cysB+ heterozygote. SCP1'-argA, uraB carries a longer chromosomal insertion: five chromosomal loci have been detected on the plasmid; it was very often lost; and there was frequent crossing-over between exo- and endogenote in heterozygous strains. Both SCP1' plasmids gave rise to plasmids closely resembling, or perhaps identical with, the original SCP1 by deletion of the chromosomal insertion. Markers argA and uraB were always deleted together from SCP1'-argA, uraB, whereas random deletion should more often have removed uraB alone. This suggests that special DNA sequences are involved in SCP1' formation. An analysis of recombination between exo- and endogenote within a double heterozygote for argA and uraB supports the hypothesis that single copies of plasmid and chromosome assort from a pool of interacting molecules at, or immediately before, spore formation. A strain carrying the SCP1'-cysB plasmid gave rise to a 'bidirectional' donor of chromosomal markers in which the plasmid had apparently integrated into the chromosome at or close to the cysB locus.