Transformation-deficient mutants of Streptococcus pneumoniae were isolated after nitrosoguanidine mutagenesis. Seventeen mutants developed normal peaks of competence, as tested by their ability to degrade one strand of donor DNA, but they yielded transformants for chromosomal point markers at efficiencies from less than 0.001 to 0.04 that of the wild type. Some of the mutants were defective in DNA uptake and are described as entry defective (Ent-). Others took up DNA in normal quantities, but they failed to give stable transformants and are described as recombination defective (Rec-). In two of the Rec- mutants, normal levels of transformation by plasmid DNA occurred; in the others, it was reduced as much as chromosomal transformation. Conjugative transfers of a chromosomal omega (cat tet) element and of the plasmid pIP501 occurred at normal levels both to and from Rec- mutants. Transfer of chloramphenicol resistance by transformation with omega (cat tet) donor DNA, however, was blocked in Rec- mutants to about the same extent as was transformation for point markers.