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This study aimed to investigate the variations of parasites in the feces of brown bears Ursus arctos inhabiting the Cantabrian Mountains (NW Spain). A total of 248 bear fecal samples were collected throughout one year, spanning from August 2018 to September 2019, at an approximate frequency of 20 samples per month. The results were analyzed in relation to both the season and the biological activity of the brown bears, i.e., hibernation, mating and hyperphagia. Among the examined samples, eggs of Dicrocoelium dendriticum (32.2%; 95% Confidence Interval: 26.4–38.1), Baylisascaris sp. (44.8%; 38.5–50.9), ancylostomatids (probably belonging to Uncinaria spp.) (16.5%; 11.9–21.1) and Trichuris sp. (1.2%; 0–2.6) were observed. Significant seasonal differences were noted for Baylisascaris and ancylostomatids (χ2 = 21.02, P = 0.001 and χ2 = 34.41, P = 0.001, respectively). Furthermore, the presence of helminth eggs was correlated with the activity phase of the brown bears. Dicrocoelium attained the highest prevalence during the mating phase, while Baylisascaris and ancylostomatids were more frequent during hyperphagia. Notably, the highest egg-output counts for Dicrocoelium and Baylisascaris sp. were recorded during the mating phase and hibernation, respectively, whereas ancylostomatids eggs peaked during hyperphagia. Additionally, variations in egg-output counts were significant for all helminths concerning the season, with the exception of Trichuris sp., and for Dicrocoelium and Baylisascaris sp. According to bear activity. It is concluded that infection by gastrointestinal helminths depends on the season and the biological activity of the bears from the Cantabrian Mountains, and their health status could result influenced. [Display omitted] • Cantabrian brown bears are infected by liver trematodes and gastrointestinal nematodes. • The lowest helminth prevalences were observed in spring and the highest in autumn. • The presence of helminth eggs correlated with the activity phase of the brown bears. [ABSTRACT FROM AUTHOR]