ABSTRACT. Freitas M.C., Coutinho-Rodrigues C.J.B., Perinotto W.M.S., Nogueira M.R.S., Chagas T.T., Marciano A.F., Camargo M.G., Quinelato S., Gôlo P.S., Sá F.A. & Bittencourt V.R.E.P.[Hemocytes quantification in Rhipicephalus microplus engorged females infected by Beauveria bassiana s.l.] Quantifica- ção de hemócitos em fêmeas ingurgitadas de Rhipicephalus microplus infectadas por Beauveria bassiana s.l. Revista Brasileira de Medicina Veterinária, 37(Supl.1):63- 70, 2015. 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: firstname.lastname@example.org Rhipicephalus microplus tick is an ectoparasite with a great negative impact for veterinary medicine. Under massive infestations cattle may present severe anemia; besides that, ticks can transmit pathogenic agents causing host death. Among the methods used to alternatively control ticks is the use of the arthropodpathogenic fungus Beauveria bassiana s.l. that has been considered promising due to the fungus capacity of penetration of full cuticle in all tick developmental stages, colonizing them and causing tick death. Arthropod immune response is basically composed of humoral and cellular components. Several cell types that actively work against microbial infection are present within the hemolymph. The aim of the present study was to evaluate the amount and identify hemocytes present in the hemolymph of R. microplus engorged females artificially inoculated with B. bassiana senso latu (s.l.) conidia (isolate CG 206). Fungal suspension (1 × 108 conidia/mL) or Tween 0.01% (control group) were inoculated in R. microplus engorged females. Hemolymph collections were made through the female dorsal region (for quantitative analysis) and the distal section of the legs (for qualitative analysis) 24, 48 and 72 hours after fungal inoculation. Hemocytes quantification was performed using a Neubauer chamber. Qualitative analysis was carried out by a smear of hemolymph on slides. Data were submitted to Mann-Whitney test for comparison of averages on a significance level of 5% (p <0.05). Comparison between unexposed females and females exposed to B. bassiana s.l. showed that hemolymph from the fungus treated group presented significant reduced amount of hemocytes 24 hours after fungal inoculation. Total amount of hemocytes 48 hours after fungal inoculation was 49.5 times lower in comparison with the amount observed 24 after fungal exposure. The following hemocytes were identified in tick females’ hemolymph: prohemocytes, granulocytes, plasmatocytes, spherulocytes and possible oenocytoids. Based on these results, it is concluded that inoculation of B. bassiana s.l. isolate CG 206 caused cellular alteration in the hemolymph of R. microplus females showing reduction of total amount of hemocytes 48 hours after fungal inoculation.