Repellency is a highly desirable attribute for an acaricide that protects dogs against the Australian paralysis tick (Ixodes holocyclus). A bite from a single tick of this species can be fatal and carries the risk of transmission of vector-borne diseases. A randomised, controlled study was conducted to confirm the repellency sensu stricto and sensu lato (expellency) of a 10 % imidacloprid/4.5 % flumethrin collar (Seresto ® , Bayer) on dogs, against artificial infestations of I. holocyclus. Based on a novel protocol, dogs treated with Seresto ® , or a placebo collar, were sedated and placed in infestation chambers for 1 h at varying time points up to 6 months after treatment. Ticks were released alongside the dogs, and allowed to approach and/or attach. Treating dogs with a Seresto ® collar resulted in significantly (p < 0.001) more residual ticks being found in the infestation chambers (repellency sensu stricto) compared to the placebo treated dogs for the 168-day duration of the study (range 67.5% to 90.2 %). Repellency sensu lato (expellency) was reflected in significantly (p <0.002) fewer ticks being found attached to treated dogs throughout the study. Comparative decrease in ticks attached exceeded 95 % for 84 days, when assessed after 6 h. When assessed after 24 h, there was a > 95 % comparative decrease in ticks attached until Day 84, > 90 % on Days 112 and 140 then > 95 % to Day 168. Efficacy (killing effect) of the Seresto ® collar based on total live tick counts was > 95 % 6 h after tick challenge throughout the 6 month study period. This study demonstrated the excellent repellency effect of the Seresto ® collar against I. holocyclus in dogs. S146Ectopar asitEs