The resistance of strongyles to pyrantel pamoate has been reported in publications worldwide. There is no data on its efficacy in horses in Poland. Therefore, the aim of this study was to evaluate the effectiveness of pyrantel pamoate to control strongyle invasions in young horses. The study involved horses of both sexes, aged 1 to 2 years, and was conducted in two seasons (spring and the end of summer). Feces were collected 24 hours before and 14 days after deworming, and they were then examined using McMaster’s and combined sedimentation-flotation methods. Results revealed that in spring, the average eggs per gram (EPG) before deworming was 1354.54, but after the treatment EPG was 485, and fecal egg count reduction test (FECRT) = 67.45%. Considering the age groups of the horses, the average EPG for one-year-olds was 1565.38, and 1050 for two-year-olds, respectively. The FECRT for strongyle invasions was 69.78% in one-year-old and 62.43% in two-year-old horses. At the end of summer, the average EPG was 1954.54 before deworming, and after the treatment, strongyle eggs were found in only two horses, and FECRT was 99.53%. Considering the age groups of horses before deworming, the average EPG in one-year-olds was 2103.85, but was 1738.89 in two-year-old horses. The FECRT values were 99.27 and 100%, respectively. The results revealed a quite limited efficacy of pyrantel pamoate in the deworming of Strongylidae invasions in young (one- and two-year-old horses) during the spring season. According to WAAVP recommendations, this was assumed to be the result of pyrantel pamoate resistance of Strongylidae nematodes in horses. In contrast, treatment using the same compound in the same age horses, but conducted at the end of summer, was found satisfactory. This was the first study concerning pyrantel pamoate efficacy in horses in Poland.