From 2014 to 2016, GE Renewable Energy and California Ridge Wind Energy tested an ultrasonic bat deterrent system during the autumn bat migration period at an operating wind farm in Illinois, USA. The deterrent system consisted of air‐jet ultrasonic emitters mounted on nacelles and towers in a different configuration each year. Each year we conducted a randomized block experiment to determine whether the acoustic deterrent reduced bat mortalities at the wind farm. Effectiveness was based on estimates of bat mortalities during 3‐day trials. The operation of the acoustic deterrent resulted in significant overall bat fatality reductions of 29.2% ( = 7.5%) and 32.5% ( = 6.8%) in 2014 and 2015, respectively. All‐bat fatality rates were not reduced in 2016; however, annual all‐bat effectiveness estimates were influenced by species composition. We analyzed deterrent effectiveness for eastern red (Lasiurus borealis), hoary (Lasiurus cinereus), and silver‐haired (Lasionycteris noctivagans) bats, the 3 species most commonly found during the carcass searches. Hoary bats were consistently deterred each year, but annual deterrent effectiveness varied for eastern red and silver‐haired bats.