Eleven Acadian families with Friedreich ataxia (FA) who were from southwest Louisiana were studied with a series of polymorphic markers spanning 310 kb in the D9S5-D9S15 region previously shown to be tightly linked to the disease locus. In particular, three very informative microsatellites were tested. Evidence for a strong founder effect was found, since a specific extended haplotype spanning 230 kb from 26P (D9S5) to MCT112 (D9S15) was present on 70% of independent FA chromosomes and only once (6%) on the normal ones. There was no evident correlation between haplotypes and clinical expression. The typing of an additional microsatellite (GS4) located 80 kb from MCT112 created a divergence of the main FA-linked haplotype, generating four minor and one major haplotype. A similar split was observed with GS4 in a patient homozygous for a rare 26P-to-MCT112 haplotype. These results suggest that GS4 is a flanking marker for the disease locus, although other interpretations are possible.