The aim of the present paper was to evaluate the larvicidal effect of <i>Enterococcus faecalis</i> CECT7121 (Ef7121) on the <i>Toxocara canis</i> cycle both in vitro and in vivo. For the in vitro experiments, <i>T. canis</i> larvae were incubated with the supernatants of <i>Ef</i>7121 (EI) and mutant <i>Ef</i>7121 (EIm), in a pre-culture of <i>Ef</i>7121 (EII) and in a fresh culture with <i>Ef</i>7121 (EIII) and the <i>Ef</i>7121 mutant strain (EIIIm). The viability of the larvae was calculated after a 48 h incubation. A significant reduction of the viability of <i>T. canis</i> larvae was observed in EI, EII and EIII. A decrease of this inhibitory effect was observed in EIm and EIIIm (p = 0.008). In the in vivo experiments, mice were orally inoculated with three doses of <i>Ef</i>7121. To study the probiotic persistence in the intestine, the animals were sacrificed every four days and their intestines were dissected. The initial average bacterial levels were 9.7 × 10 4 for <i>Ef</i>7121 (colony forming units/g). At the end of the assay the levels were 1.46 × 10 4. No bacterial translocation was detected in mesenteric lymphatic nodules and spleen. <i>Ef</i>7121 interference with the biological cycle was evaluated in mice challenged with <i>T. canis</i>. The interference was significant when the mice were challenged with probiotic and <i>T. canis</i> simultaneously (p = 0.001), but it was not significant when the challenge was performed 15 days after administration of the bacterial inoculum (p = 0.06). In conclusion, <i>Ef</i>7121 possessed in vitro and in vivo larvicidal activity.Facultad de Ciencias Médicas