Mating pattern and gene flow were studied in the contact zone between two morphologically very similar Chiffchaff taxa (Phylloscopus collybita, P. brehmii) in SW France and northern Spain. Mating was assortative in brehmii, but not in collybita. Mixed matings were strongly asymmetric (excess of callybita male x brehmii female pairs), but did produce viable offspring in some cases. Sequence divergence of the mitochondrial cytochrome b gene was 4.6%; Haplotypes segregated significantly with phenotype (only five 'mismatches' among 94 individuals), demonstrating that mitochondrial gene flow was very restricted. The estimated proportion of F-1 hybrids in the reproductive population was significantly lower than expected under a closed population model, indicating strong selection against hybrids. Genetic typing of 101 individuals at four microsatellite loci also showed significant population differentiation, but nuclear gene flow was estimated to be 75 times higher than mitochondrial gene flow. This strong discrepancy is probably due to unisexual hybrid sterility (Haldane's rule). Thus, there is a strong, but incomplete, reproductive barrier between these taxa.