Objective The objective of this study was to determine the repellency and efficacy of a 10% imidacloprid/4.5% flumethrin (Seresto ® , Elanco) collar over an 8-month period against the eastern paralysis tick (Ixodes holocyclus) on cats.Methods Two non-blinded, open gender, randomised, placebocontrolled pen studies were conducted, with 26 cats enrolled in each study. Prior to inclusion, cats were immunised with I. holocyclus holocyclotoxin. Cats were treated on Day 0 with either an imidacloprid/flumethrin or placebo collar. Tick infestations with 20 unfed adult female eastern paralysis ticks commenced on Day 7, and were repeated monthly for 8 months. Repellency was determined by comparing the mean number of attached ticks on imidacloprid/flumethrin treated cats, to placebo collar treated cats at 6 and 24 h post infestation. Efficacy was determined by comparing the mean number of live ticks on imidacloprid/flumethrin collar treated cats to placebo collar treated cats at 72 h post infestation. ResultsEfficacy was 100% (P < 0.001) at 72 h, and repellency was greater than 96% (P < 0.001) at 24 h for every tick challenge in each of the two studies, from Day 7 to the final infestation at 8 months for imidacloprid/flumethrin collar treated cats.Conclusions In two pen studies, an imidacloprid/flumethrin collar controlled and repelled the eastern paralysis tick (I. holocyclus) on cats for 8-months. The marked repellency effect in addition to controlling tick paralysis would be beneficial in preventing tick bites and their sequelae.