Macrolide resistance associated with macrolide efflux (mef) in Streptococcus pneumoniae has been defined with respect to the genetic structure and dissemination of a novel mefE-containing chromosomal insertion element. The mefE gene is found on the 5'-end of a 5.5 kb or 5.4 kb insertion designated mega (macrolide efflux genetic assembly) found in at least four distinct sites of the pneumococcal genome. The element is transformable and confers macrolide resistance to susceptible S. pneumoniae. The first two open reading frames (ORFs) of the element form an operon composed of mefE and a predicted ATP-binding cassette homologous to msrA. Convergent to this efflux operon are three ORFs with homology to stress response genes of Tn5252. Mega is related to mefA-containing element Tn1207.1. Macrolide resistance due to mega has been rapidly increased by clonal expansion of bacteria containing it and horizontally by transformation of previously sensitive bacteria.