Although umbels of common hogweed (<i>Heracleum sphondylium</i>) are visited by numerous insect species, only about 2-3 taxa pollinate its flowers. This paper presents two-seasonal observations of diurnal activity of the major insect visitors to inflorescences of <i>H. sphondylium</i> and its key pollinators. The results obtained for two <i>H. sphondylium</i> subspecies occurring naturally in Poland show that insect activity is the highest in the first part of the day and decreases towards the

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... vening. This is mainly implicated by the behavior of dipterans, which intensively feed on hogweed flowers. Other insect orders (Coleoptera, Hymenoptera), but also the dipteran key pollinators (<i>Eriozona syrphoides</i> and <i>Lucilia</i> spp.) do not express any statistically significant preferences for any part of the day.