Abstact. The objective of the present study was investigate the prevelance of Gasterophilus sp. larvae in Poland and characterize the parasitic invasion during one year. From the January to December 2006 years 513 horses were examined. On the whole 250 horses were infected by larvae of Gasterophilus sp. The mean prevalence of invasion was 48,73%. The highest monthly prevalence observed in November. Of the 33 horses examined 23 were infested with Gasterophilus sp. The mean intensity of larvae was 67, the highest was in January – 110. Two species of Gasterophilus were identifited with the following prevalence: G. intestinalis – 216 horses and G.nasalis – 8 horses. The 26 horses were infested by both species G. intestinalis, and G.nasalis. Prevalence and intensities were not affected by age, and sex. The larvae were found in the stomach and duodenum of adults and foals.

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