Comparative analysis of heteroduplex patterns on the STR FES/FPS system led to the detection of a new non-consensus allele 10 in the African population of São Tomé e Príncipe. Automated sequencing confirmed a T-->C substitution at position 177 (1stT of the 6th repeat) which was exclusively found in haplotypic combination with base A in the previously described polymorphic position 34. The new substitution was not detected in a sample from North Portugal. Sequence analysis revealed a triplet of inverted bases, from positions 101 to 103, relative to the sequence described in the GeneBank (Accession No. X06292). This work confirms the capacity of heteroduplex analysis in the detection of DNA structural microvariation and emphasises the complementary utility of manual system analysis and semi-automated techniques for a full characterisation of the genetic variability of STRs.