The experiments were carried out on 285 cows from 10 herds from different regions of Poland. Extensity of the H. bovis invasion in each herd varied from 10 to 86%. The highest extensity appeared in herds from eastern and north-eastern Poland and the lowest in herds from southern and central regions of the country. Seroconversion of Hypoderma bovis antibodies was calculated for animals in which warbles were detected by clinical examination. The highest serum densities were observed in herds in March, April and May.