Pollen flow in the heterostylous species P. obscura was studied in two localities in Denmark. Various floral characters of the two morphs were measured, but differences were found only in height of stamens and stigma, and size of pollen. In studies with a scanning electron microscope (s.e.m.) pollen was discovered to have a continuous, equatorial girdle formed by the transverse furrows extending laterally from the ora. The stigmatic papillae were composed of a stalk with a many-armed crown. The ratio of number of thrum to pin plants was approximately 1:1 fulfilling theoretical expectations. Six different Bombus spp. and a specimen of Psithyrus bohemicus were observed as visitors to P. obscura, which displays a range of adaptations to Bombus pollination. Thirty per cent of the Bombus specimens had corbicular pollen loads, which were almost 100% pure Pulmonaria pollen. Pollen flow was revealed by analysing the pollen loads on stigmas and proboscides. Both stigma types had more illegitimate pollen than expected on the basis of the ratio of production of the two pollen types. This could be caused by a high degree of intrafloral pollen flow. Since heterostyly does not apparently enhance the transport of legitimate pollen the adaptive role of the system is unclear.