Phenological observations of development of generative organs of Pinus sylvestris and P. uliginosa (= P. rotundata) were carried out in spring of 1999, 2000 and 2001. The aim of the research was to verify the hypothesis that both taxa have a possibility of reciprocal crossing. Two populations of P. uliginosa were tested, one on the Silesian Lowland (Węgliniec) (altitude of 180m), the other in the Stołowe mountains (Batorów) (at altitude of 750 m). The lowland population of P. uliginosa is not numerous and surrounded by extensive P. sylvestris forests, the mountain one is 3 times more numerous, and the nearest population of P.sylvestris is at a distance of 2 km. Observations in the field were carried out twice a week during the period of development of micro- and macro-strobili of both taxa. The results show that the P. sylvestris strobili developed every year earlier than those of P. uliginosa. Nevertheless, this did not eliminate the possibility of cross pollination and gene flow from P. sylvestris toward P. uliginosa on both localities of the last species. The possibility of pollination of P. sylvestris macrostrobili by P. uliginosa pollen was found also possible, but more restricted. Normally, on lowlands the same phenological phases take place about 10 days earlier than in the mountains. This is documented for two years of observations of more or less typical whether conditions. An earlier development of micro- and macrostrobili of both species in their mountain populations was observed in an observation year characterized by an extremely early and warm spring.