ey meet in Eastern Europe, generally along the edge of upland regions. B. variegata inhabits small, ephemeral puddles in hilly and mountainous areas, whereas B. bombina is found in flooded grasslands or ponds in the lowlands. They differ for many traits, most notably belly pattern, mating call, larval development time and adult skeletal morphology (Sanderson et al., 1992; Nürnberger et al., 1995) .
             The relative proportions of either parent taxa in a hybrid toad can be summarised in several ways. Firstly, the number of joins between the spots in the belly pattern can be used as a simple measure of hybridity (Szymura & Barton, 1986 and references therein) . The number of variegata alleles in diagnostic marker loci can also be summed across loci to give a hybrid index. In Croatia and Poland, the relationship between these two is good, because there are generally high levels of genome wide linkage disequilibrium (D) resulting from the dispersal of pure individuals into the zone.
             Previous work in Croatia has shown that hybrids between these taxa are unfit (both in terms of egg viability and survival: Kruuk & Gilchrist, 1997), showing some level of hybrid inviability. This is also supported by selection coefficients derived using cline theory (Barton & Gale, 1993) , for sites in both Poland and Croatia (Szymura and Barton, 1986; Szymura & Barton, 1991; MacCallum et al., 1998) . The hybrids in Croatia also show a habitat preference in line with the taxa they most resemble, reducing the contact rate between hybrids from either side of the zone, even when the two habitat types are in close proximity (MacCallum, 1994) .
             This study will focus on the Bombina hybrid zone in the Carpathian mountains of Romania, where the toads occur in high numbers over a wide area. The obviously different structure of the zone here requires that data on hybrid fitness and habitat preference must also be collected so that quantitative compari...