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Insertion-duplication mutagenesis (IDM) can identify a high proportion of both dispensable and essential genes. This was found following the construction of a collection of chimeric plasmids with 300-bp inserts, which is both highly complex and highly mutagenic. The mutagenic plasmids were evaluated for size uniformity and randomness of targeted sites by sequencing randomly selected plasmids. Their collective utility as a mutagenic library was investigated by characterizing mutants obtained by integration into the pneumococcal chromosome. IDM was found to be a useful tool for genomic analysis since it hit most known competence genes repeatedly and mutated an additional competence gene previously unidentified via random or specific mutation methods.