It has been proposed that sucking by lambs is associated with transfer of microorganisms into the teat. There are three different sources of bacteria for the teat duct, namely the lamb (mouth, nasopharynx), the ewe (udder skin) or the environment (bedding). The bacteria can ascent to the mammary gland and cause mastitis. Objectives of the study were i) to determine differences in bacterial flora populations in the teat duct and mammary secretion of ewes before and after sucking and ii) to evaluate factors which affect these differences.