This study was conducted in the grasslands of the Department of Casanare, seeking to determine the degree of anthelmintic susceptibility or resistance against macrocyclic lactones in small populations of Strongylus in this region of the country. Samples were taken from four municipalities in the department: Aguazul, Paz de Ariporo, Mani and El Yopal, between June, 2006 and April, 2007, where ten fresh fecal samples were collected in each municipality, directly from the field and in a completely random way. The samples were processed using the MacMaster coprological technique, thus determining the highest count of fecal epg by municipality. L3 larvae were extracted from each of the six samples with the highest count Determinacion de los grados de resistencia antihelmintica que presentan los pequenos strongylus frente a las lactonas macrociclicas, segun el test de analisis del desarrollo de larvas (LDA) en equinos de las sabanas de Casanare 1 Medico veterinario, Universidad Nacional. M.Sc., Universidad Austral de Chile. Docente de la Facultad de Ciencias Agropecuarias, Universidad de La Salle.  geprada@unisalle.edu.co. 2 Medica veterinaria Universidad de La Salle. M.Sc. (c) en Fisiopatologia Veterinaria, Universidad Nacional de Colombia. Docente de la Facultad de Ciencias Agropecuarias, Universidad de La Salle.  csromeroh@unal.edu.co.