Seven dairy-goat farms from the centerwest of france were investigated for morphology and ecology of first-stage larvae (L1) of Muellerius capillaris before and after treatment of goats with febantel, a probenzimidazole anthelmintic. The lengths, survivals at 20 and -20 degrees C, and infectivity of L1 to intermediate host Helix aspersa, were different between farms. The between farms differences in survivals were reduced after treatment of goats with febantel. The observed between farm differences in L1 did not seem to be related to farm characteristics (intensity of treatments, susceptibility to febantel, and intermediate host species).