In this report, we describe a transposon (TN1000)-mediated mutagenesis method for the generation of random, in-frame mutations in target sequences. Rather than single nucleotide changes, the mutations generated by this method are replacements of three nucleotides with GGG or CCC. The effectiveness of this method was confirmed on pBluescript SK (+) DNA and the efficiency of each step is optimized. The mutagenesis method described here would be very useful for fine mapping of important gene sequences, particularly when numerous random, in-frame mutations are desired or when single nucleotide changes are expected to revert frequently.