Capsule phase variants were isolated from serotype 8 and serotype 37 pneumococcal sorbarods. Sequence duplications within the essential capsule genes - cap8E (type 8) and tts (type 37) - were found to introduce frameshifts and generate acapsular phenotypes. Capsular revertants possessed wild-type cap8E and tts genes, indicating the precise excision of these duplications. Reversion frequencies (OFF-ON) fit a linear relationship between log(frequency of reversion) and log(length of duplication), previously found for serotype three pneumococci [Waite, R. D., Struthers, J. K. & Dowson, C. G. (2001). Mol Microbiol 42, 1223-1232]. This study provides evidence that capsule phase variation can occur in pneumococcal serotypes with either simple (one to three genes) or complex capsule-encoding loci (12 genes). Given the key role of CapE (the first monosaccharide transferase) in other clinically important pneumococci, such as serotypes 14 and 19F with complex capsular loci, the observed duplication within cap8E suggests that capsule phase variation could be controlled by tandem sequence duplication in capE homologues in other pneumococcal serotypes that construct their capsules through polymerization of lipid-linked intermediates.