The Streptococcus faecalis transposon Tn917 was introduced into Bacillus subtilis by transformation of competent cells with the plasmid pAM alpha 1::Tn917 and was tested for transposition activity by selection for insertions into the temperate phage SP beta. Insertions were obtained at a frequency indicating relatively efficient movement of the element, and Southern hybridization analysis of a particular insertion confirmed it to be the result of a genuine transposition event. A restriction fragment from pAM alpha 1::Tn917 containing the transposon sequences was ligated into a temperature-sensitive plasmid (pBD95), and transpositions into the B. subtilis chromosome were selected by requiring the transposon drug resistance to be maintained at temperatures nonpermissive for plasmid replication. Insertions have been recovered at many chromosomal sites, including ones that produced auxotrophy of different kinds and ones that produced various different sporulation-defective phenotypes, indicating good prospects for the use of Tn917 as a tool for insertional mutagenesis in B. subtilis.