AbstractParasitism by gastrointestinal nematodes is a challenge for small ruminant farming worldwide. It causes productive and economic losses, especially due to parasite resistance to conventional anthelmintics. Natural compounds with antiparasitic activity are a potential alternative for controlling these parasites especially when considering the widespread occurrence of anthelmintic resistance. Our objective was to evaluate the activity of anacardic acid, geraniol, cinnamaldehyde and citronellal onHaemonchus contortusisolates with different levels of anthelmintic resistance profiles. These compounds were tested using egg hatch assays (EHAs), larval development tests (LDTs) as well as LDTs on mini-fecal cultures, on theHaemonchus contortusisolates Kokstad (KOK-resistant to all anthelmintics), Inbred-Strain-Edinburgh (ISE-susceptible to all anthelmintics) and Echevarria (ECH-susceptible to all anthelmintics). Effective concentrations to inhibit 50% (EC50) and 95% (EC95) of egg hatching and larval development were calculated. Results for EHA and LDT for all tested compounds, considering EC50and EC95values, showed low variation among the studied isolates with most RF values below 2x. All studied compounds showed efficacy against egg hatching and larval development ofH. contortusisolates regardless of anthelmintic resistance profiles. The compounds with the smallest EC50and EC95values were cinnamaldehyde and anacardic acid making them promising candidates for futurein vivostudies.