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ABSTRACT. Marciano A.F., Fiorotti J.P., Souza L.A., Camargo M.G., Perinotto W. M.S., Angelo I.C., Gôlo P.S., Sá F.A., Coutinho-Rodrigues C.J.B., Quinelato S. & Bittencourt V.R.E.P. [Efficiency in vitro of a oil formulation of Metarhizium anisopliae sensu lato in the control of Rhipicephalus microplus.] Eficiência in vitro de uma formulação comercial de Metarhizium anisopliae sensu lato no controle de Rhipicephalus microplus. Revista Brasileira de Medicina Veterinária, 35(Supl.2):28-34, 2013. Departamento de Parasitologia Animal, Instituto de Veterinária, Anexo 1, Universidade Federal Rural do Rio de Janeiro, Campus Seropédica, BR 465 Km 7, Seropédica, RJ 23897-970, Brasil. E-mail: email@example.com Rhipicephalus microplus is an ectoparasite responsible for great economic losses in cattle industry. In an effort to reduce the use of chemical acaricides, which causes many environmental problems, the research for alternative ways to control ticks, in special the use of formulations of entomopathogenic fungi, has been attracting attention. The aim of this study was to evaluate the effectiveness of oil-based formulations in different oil concentrations of a commercial product of Metarhizium anisopliae (Metarril WP Organic®) against eggs, larvae and engorged female of R. microplus. Eight groups were formed: an aqueous control group treated only with sterile distilled water and 0.1% Tween 80, a group treated with the aqueous suspension Metarril WP Organic®, three control groups containing 1, 3 or 5% mineral oil and three groups treated with the oil-based formulations Metarril WP Organic® containing 1, 3 or 5% mineral oil. The formulations had a concentration of 108 conidia.mL-1. The treatment consisted of the immersion of the specimens in one mL of suspension tested for three minutes. The biological parameters of engorged females were significantly affected by the oil-based formulations Metarril WP Organic®, leading to 97.35% efficacy control in the group treated with the fungal oil-based formulation containing 5% mineral oil. In the treatment of eggs, oil-based formulations Metarril WP Organic® decreased 55 times larvae hatching when compared to the control groups. In the treatment of larvae mortality was 100% at fifth day after the treatment with the fungal formulations containing 3 and 5 % mineral oil. These results suggest that the formulations of Metarril WP Organic® containing 1, 3 or 5 % mineral oil are effective in vitro against tick developmental stages of R. microplus.