A hybrid zone will be formed when diverged populations meet and interbreed, and give rise to viable hybrids. When these are also fertile, further generations of backcrossing can produce a wide range of hybrid classes. If the zone is stable through time, then the strength of the different mechanisms slowing gene flow can be determined, and thus their relative importance in keeping the parental populations distinct. In this way, hybrid zones can provide much data on how populations with differing genetic architectures interact (Barton & Hewitt, 1989) .
             There are two forms of gene flow barriers in hybrid zones, and both broadly act to prevent the formation of intermediate hybrids, which can then reproduce. Firstly, the hybrids may be unfit, and thus have a lower chance of surviving to mate. Selection against hybrids can either be against recombined genomes, perhaps due to the break-up of co-adapted gene complexes, or against alleles in the wrong environment, in which case fitness depends on the location of the hybrid (Barton & Hewitt, 1981; Barton & Hewitt, 1985) . Secondly, hybrids may achieve fewer matings, and thus have a lower reproductive success than parental genotypes.
             The level of assortative mating between hybrids with different genetic backgrounds is also important in determining the rate of formation of intermediate hybrids, and this has two aspects. Firstly, the level of habitat preference among hybrids (i.e. are they found with the parent taxa they most resemble?) will affect how often different classes can meet in a 'reproductive context'. The second aspect is the frequency within habitats of matings between hybrids with very different genetic constitutions.
             This study aims to describe mating patterns in the hybrid zone between the toads Bombina bombina and B. variegata, so that their importance relative to other processes (such as hybrid unfitness) can be determined. These two taxa hybridise wherever th...