In an experimental flock of Polish long-wool sheep (PL), rams and ewes were progressively replaced by Blackface (Bl) and Weisses Alpenschaf (WA) sheep. The ewes and their lambs were naturally infected with predominantly Teladorsagia circumcincta and Haemonchus contortus nematode species. A survey was done from 1995 to 2000 by regular FECs. Significant fixed effects were: the timing of last anthelmintic treatment, month and year of sampling in ewes and lambs, plus the age (yearling or not) and the suckling effort for the ewes as well as the sex of the lambs. The breed effect was highly significant. PL ewes excreted three times fewer eggs than Bl. Also PL and WA lambs possessed 1.5 fewer nematode eggs in their faeces than Bl ones. It is concluded that introduction of Bl and WA for breeding will require more careful control of nematode infection than the management of the local long-wool breed.