In suitable habitats the population density of the Dice Snake (Natrix tessellata) can be obviously higher than the density of other central European snakes. Under such conditions the Dice Snakes can form larger aggregations of mating individuals, in which male-male mating competition takes place. Dice Snake mating aggregations observed in the Ohře river valley are described and the mating strategy of males and female is discussed. Special attention is paid to mating in above-ground positions, e.g. on trees. and Jiří Moravec.