To elucidate the molecular mechanisms of macrocyclische lactone-resistance in the bovine parasite Ostertagia ostertagi, the genetic polymorphism in the P-glycoprotein A, glutamate-gated chloride channel subunit alfa 3 and beta-tubulin isotype 1 genes will be investigaed in ML-susceptible and resistant isolates of O, ostertagi, a Molecular tests will be developed to detect specific mutations or increased transcription levels associated with ML-resistance.