Genetic plasticity plays a central role in the biology of the human pathogen Streptococcus pneumoniae. This is illustrated by the existence of at least 90 different capsular types (the polysaccharide capsule has an essential antiphagocytic function) as well as by the rapid emergence of penicillin-resistant (PenR) pneumococcal isolates. Natural genetic transformation is believed to be essential for this genetic plasticity; capsular types can be switched by intraspecies transformation, whereas interspecies transformation is responsible for the appearance, in the PenR isolates, of mosaic pbp genes, which encode proteins with reduced affinity for penicillin. Data on the regulation of competence for transformation in S. pneumoniae, on the control of intra- and interspecies genetic exchange and on the shuffling and capture of exogenous sequences during transformation are reviewed. Possible links between transformation and changes in environmental conditions are discussed, and the adaptive 'strategy' deduced for S. pneumoniae is compared with that of Escherichia coli.