Summary.Haemophilia B is due to multiple molecular defects in the factor IX gene. Most of them are single base substitutions, and can now be identified by direct sequencing of the coding sequence of the factor IX gene, preceded or not by a screening strategy. In some instances the mutation alters an enzyme recognition site and this allows rapid and accurate carrier testing and prenatal diagnosis in the affected pedigree. This was not the case for the previously described nt 31119 (G–>A) mutation that we found in an extended haemophilia B pedigree, during the search for mutations in the factor IX gene in patients from Southern France. We first detected this mutation by single stranded conformation polymorphism (SSCP) and then identified it by DNA sequencing. Carriership could be easily determined in the females of the pedigree by analysis of the SSCP patterns. Our results indicate that the SSCP analysis of amplified genomic DNA fragments can be successfully used as a diagnosis approach for direct carrier testing and prenatal diagnosi