In the years 1976-1989 392 nests of D. urbica and 584 nests of H. rustica were collectcd from 54 localitics in 6 Provinces in the north of Poland. The highest mean number of specimens per nest was observed in the nest of D. urbica (28.4) and the lowest in nest of H. Rustica (4.2). The highest percentage of nests of the swallows with Oe. hirundinis was recorded in December and lowest in March. The highest mean number of Oe. hirundinis per nest was in the nest of D. urbica in August and lowest in February, and in the nest of H. rustica - in September and in March respectively.