To achieve a better understanding of the genetics of capsular polysaccharide synthesis in Streptococcus pneumoniae, we have identified and characterized mutants deficient in type 3 capsule production. We identified a clone that restored encapsulation in one of our mutants and in a mutant deficient in UDP-glucose dehydrogenase. By hybridization, we developed a chromosomal map of the type 3-specific region and identified a flanking region containing DNA common to all capsule types examined. Insertion mutations were used to identify chromosomal loci required for capsule synthesis, and to map transcription within the region. Using non-destructive insertions linked to type-specific genes of type 2, 3, or 5, we were able to select for the transformation of all necessary genes specific for capsule type. Our data provide molecular evidence to show that all the type-specific genes are linked in a cassette and can be transferred as a unit during transformation.