Abstract The aim of the present study was to evaluate the effect of an anthelmintic treatment in ewes ( n = 133) of two German sheep breeds (German Black Head Mutton and Leine sheep) on the average daily weight gains (ADW) of their lambs ( n = 180). Eight weeks prior to the lambing season ewes were randomly divided into two groups: (1) no anthelmintic treatment, (2) Moxidectin (Cydectin ® , Forte Dodge, USA) treatment. Starting at birth, body weights of lambs were recorded every other week until 12 weeks of age. Ewes faecal egg count (FEC) was measured 8 weeks after lambing to monitor the efficiency of anthelmintic treatment. Log FEC was significantly lower ( P P = 0.212) to have higher log FEC compared to Leine sheep (3.08 ± 0.29 vs. 2.46 ± 0.39). Average daily weight gains were significantly higher in German Black Head lambs compared to Leine lambs ( P P P Gastrointestinal nematode infections tended to be less severe in Leine sheep. The anthelmintic treatment did not influence growth rates of Leine lambs, suggesting a potentially higher nematode resistance or tolerance in this breed. In German Black Head Mutton, single born lambs from Moxidectin-treated ewes had higher growth rates compared to single born lambs from untreated ewes, whereas no differences could be observed between lambs from treated and untreated ewes that had more than one lamb. These findings may be a consequence of differences in intake of milk, concentrate, and hay.