In Salmonella typhimurium, insertion of transposons Tn5, Tn10 and bacteriophage Mu is inhibited by transcription of some target sequences. The transcription effects on Tn5 are large when the lac operon is a target but are limited to a slight effect on the hisG gene of the his operon. The Tn10 element shows target occlusion in both operons. Phage Mu has been shown previously to be inhibited for insertion into the lac operon. In the his operon Mu is only inhibited for insertion into the hisG gene. The variability of the inhibition effect from one sequence to another suggests site or regional specificity for transcription effects. Reducing the probability of insertion into transcribed sequences may be of selective importance to transposons since it reduces the risk of killing the host while maintaining the ability to transpose.