A convenient system for detection of deletion formation in the DNA containing Tn3 is described. EcoRI-D fragment containing the kil gene and the cI857 gene of λcI857S7 DNA was ligated into pYS1 (a colicin E1 like plasmid) containing Tn3. Bacteria harboring these plasmids were temperature sensitive due to induction of the kil gene of phage λ at high temperature. Survivors from these bacteria at 42°C have plasmids that have lost the kil gene activity by deletion. In certain mutants in Tn3 which we isolated, such deletion was not detected, nor was Tn3 transposition. On the other hand, in another mutant, in which BamHI site located in tnpR gene in Tn3 was eliminated, the frequency of deletion formation was enhanced, while transposition was normal. From these results, deletion can be considered to be a partial reaction of transposition.