The position of junctions and the extent of the duplicated chromosomal regions in Bacillus subtilis merodiploid strains were studied by quantitative DNA-DNA hybridization. We describe a method which allows (i) the identification of genes present in two copies per chromosome and (ii) the measurement of the amount of additional DNA in chromosomes with relatively large duplicated regions (about 10% or more). Analysis of previously described B. subtilis merodiploid strains GSY1127, GSY1800 and GSY1835 revealed that the duplicated segments represent 29 +/- 2%, 7 +/- 2% and 13 +/- 2% of the chromosome, respectively. Small discrepancies between these and previous genetic linkage data are discussed. Support for a role of prophage SP beta in the formation of merodiploid GSY1835 is provided. In conclusion, the described method confirmed the genetic maps of the merodiploids previously obtained by transduction and transformation crosses and showed that a duplication of a segment is not accompanied by large deletions of other chromosomal regions, providing direct evidence that a cell can accommodate genomes of substantially increased size.