Abstract The anthelmintic effects of flubendazole (FLU), its two main metabolites reduced flubendazole (FLU-R) and hydrolyzed flubendazole (FLU-H), and thiabendazole (TBZ) were compared using an in vitro larval development test in two isolates of Haemonchus contortus, a fully susceptible isolate (HCS) and a multi-resistant isolate (HCR). Results were quantified as 50 % lethal concentration (LC50), 99 % lethal concentration (LC99), efficacy factor (EF), and resistance factor (RF). For HCS, both LC50 and LC99 of FLU were lower than those of the reference TBZ. The anthelmintic activity of FLU-R in HCS and HCR was 13 and 6 times lower than the activity of FLU, respectively. The anthelmintic activity of FLU-H was negligible (approximately 363–853 times lower) compared to that of FLU. Although a marked resistance of the HCR isolate to TBZ was confirmed, only a low tolerance to FLU-R and slightly higher tolerance to FLU were found.