The present trial was aimed to compare efficiency of prevention of moniaziasis (Treatment Programme 1, TP1) versus prevention of gastro-intestinal strongylosis (Treatment Programme 2, TP2) in suckling lambs as assessed by clinical appearance and growing performance. Clinical observations and fecal egg counts were conducted on 397 three-month-old lambs born in the middle of the lambing season. Clinical signs suggestive of gastro-intestinal parasitism were found in 59 lambs (14.9 percent) and 150 out of 213 analysed fecal samples (70.4 percent) contained eggs of Moniezia. The twelve farms involved in the trial were paired following similarities in husbandry conditions. Ninety lambs from the farms under TP1 were compared with 90 lambs from the farms under TP2, following similarities of weight and sex. In the farms under TP1, the lambs received a single dose of niclosamide, 80 mg/kg bodyweight (drug active against Moniezia only). In the farms under TP2, the lambs received a single dose of morantel tartrate, 8 mg/kg bodyweight (drug active against gastro-intestinal nematodes only). Clinical symptoms, as assessed during two control visits carried out one and two month after the treatment, disappeared in farms under TP1 and increased in the farms under TP2. Eggs of Moniezia were no longer found in fecal samples of the flocks under TP1 whereas the percentage of animals with eggs of Moniezia stabilized in the flocks under TP2. Mean daily gains in the month following the treatment were 301 g in the flocks under TP1 and 128 g in the flocks under TP2. Mean daily gains in the next month were 178 g and 112 g in both groups, respectively. These results suggest that, in the conditions under study where gastro-intestinal strongylosis is controlled in the adult sheep, prevention of monieziasis must be regarded as the basis of the Herd Health Programme in the growing lambs.