A standard electroporation procedure was utilized to introduce a range of Gram-positive plasmid vectors into nine industrial strains of Streptococcus thermophilus. All the strains were transformable with at least two of the plasmids assessed, but electrotransformation frequencies depended on both the strain and the nature of transforming DNA. In general, small rolling circle (RC) plasmids could be electroporated at high frequency into a wide range of strains with efficiencies of 102–105 transformants μg−1 of transforming DNA. The presence of these plasmids did not influence doubling times during growth in broth, and they were generally extremely stable in slow milk acidifying strains, with 85–100% of transformants retaining the selective markers over 105 generations. Vectors were less stable in fast-growing cultures. Of the three theta-type plasmids assessed, only one, pIL253, could be electroporated at low frequency into some slow growing strains. The presence of this plasmid caused a 40% increase in doubling time and it was lost from cells at a rate of 3% per generation. Attempts to alter the proteolytic status of slow acidifying strains of Strep. thermophilus by the introduction of heterologous proteinase genes are also described.