Generally acknowledged reduction of the interspecific hybrid fertility was utilized in evaluation of the putative hybrid swarms of Scots pine (Pinus sylvestris L.) and mountain dwarf pine (P. mugo Turra) viability under field conditions. Pollen viability and seed quality of the trees from the four contact zones of P. sylvestris and P. mugo in Slovakia were compared with the corresponding characteristics of the pure populations of these species from three reference localities. Pollen germination percentage was comparable in contact zones and control populations of the species. Statistically significant differences between the parent and hybrid groups were detected in pollen tube length only. In general, pollen tubes of the pure species populations were longer than those in contact zones. At the cone level, the individuals from contact zones possessed shorter cones than the trees of pure species populations P. mugo and P. sylvestris. This was paralleled by a lower number of seeds per cone in contact zones and higher amount of seeds per cone in the P. mugo population. Striking differences between individuals of the contact zones and reference populations were revealed also in germination potentials of their seeds. Significantly higher proportions of germinating seeds was found in the reference populations of P. mugo and P. sylvestris than in their contact zones. Contrary to seed quality parameters mentioned above, the fractions of filled but non-germinating seeds and amounts of empty seeds were higher in contact zones than in the parental species populations, indicating higher frequency occurrence of abortive embryogenesis in the former. The lower fertility characteristics of the individuals from contact zones indicate their hybrid nature.