Allele frequencies and a single-base substitution polymorphism for 3 short tandem repeat (STR) loci of the human myelin basic protein (MBP) gene were evaluated in North Portugal and São Tomé e Príncipe. Strong linkage disequilibrium between loci MBPB and MBPC was found. However, the patterns of nonrandom allelic associations were very different in the 2 populations: levels of haplotypic diversity and heterozygosity were higher in the São Tomé population. Similarly, a difference in the frequency of base substitution G-->A at position 124 was found: the frequency reached 4.1% in North Portugal and 0.5% in São Tomé. In both populations it was always found to be associated with haplotypes B10/C11 and B12/C9.