Conditions of electroporation to transform encapsulated strains of Streptococcus pneumoniae with plasmid DNA have been defined. For a heavily encapsulated strain, an electroporation solution of 10-20% (v/v) glycerol and 3.2 kV cm-1 field strength, 1000 OMEGA resistance and 25 muF capacitance were optimal. For lightly encapsulated or non-encapsulated strains, optimal conditions were a sucrose-based electroporation solution and 12.5 kV cm-1 field strength, 200 OMEGA resistance and 25 muF capacitance.