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Urban environments are increasingly important for biodiversity conservation, but pet cats threaten wildlife therein, displaying nuisance behaviour such as hunting, fighting, fouling and urine spraying. In an attempt to empower landholders wishing to reduce cat incursions humanely, we tested the effectiveness of two ultrasonic cat deterrents (CatStop© and On-Guard Mega-Sonic Cat Repeller©).After confirming in arena trials that cats detect and respond negatively to an ultrasonic device, we tested both deterrents in 18 suburban gardens in Perth, Western Australia. Camera monitoring at foci of cat activity (e.g. fish ponds, property entry/exit points) occurred for two weeks before (Period 1: device off), during (Period 2: device on) and after (Period 3: device off) the activation of deterrents. Data included individual cat demographics and behaviours, number of cat detections per site per day per sampling period, the duration of cat activity, and detection of non-target species.Seventy-eight unique cats were detected at 17 of 18 garden sites (2–9 cats/garden). Over half the cats could be sexed (56.4%, with 65.1% males). Nearly 53.0% of cats were confirmed to be pets living nearby. Cats that were most active in period 1 (≥100 s total activity duration) were classified as ‘residents’; all others were ‘peripherals’.Overall, the ultrasonic deterrents reduced the frequency of incursions into gardens by resident cats by 46%, while the duration of incursions was reduced by 78%. Cat activity declined significantly from period 1 (baseline) to period 2 for resident cats but not peripheral cats (50% reduction; p = 0.001), and remained depressed in period 3 for resident cats but not peripheral cats (p 0.05). Cats confirmed as owned (53% of cats) generated more activity than cats of unknown ownership status (p = 0.03), probably reflecting proximity of their residences to trial gardens. Both deterrent models had similar effects (p = 0.89).By allowing pets to roam, cat owners are complicit in cat nuisance. This requires public education. Ultrasonic deterrents offer a cost-effective, humane option to reduce incursions by unwanted cats. Ultrasonic deterrents will not prevent all incursions, but they reduce their frequency and duration. Reduced cat activity has flow-on benefits to wildlife across a variety of urban-suburban settings, including gardens and parks. Keywords: Cat, Felis catus, Garden, Nuisance activity, Ultrasonic deterrent, Roaming