Over a 4-year period, from May (1995) or June (1996-1998) to November, in a flock of Polish Heath sheep, faecal samples from 246 ewes, 9 rams and 429 lambs were examined for gastrointestinal nematode eggs by the McMaster method. Faecal cultures were run to identify the nematode species present. The prevalence of positive egg counts was, on average, 43 to 69% in ewes and 68 to 80% in lambs. Two nematode genera predominated: Haemonchus contortus and Trichostrongylus colubriformis. In the last three samplings in 1998 increased frequency of Ostertagia (Teladorsagia) circumcincta infections was noticed. Animal model restricted maximum likelihood was used to estimate the heritability and repeatability for faecal nematode egg count (FEC) and correlations with productive traits. The obtained values of these parameters were 0.128 (±0.011) and 0.068 (±0.005) for repeatability and heritability, respectively. A negative influence of faecal worm egg count on fleece weight of ewes and weight gain from birth to 3^rd month, from 3^rd month to 12^th month and during 12 months of lambs' life was observed. The estimated genetic correlations were: -0.014 (±0.053), -0.197 (±0.046), -0.204 (±0.073), and -0.174 (±0.074), respectively.