Increasing instances of herbicide-resistant annual bluegrass have limited turf manager’s options for chemical control. Endothall inhibits serine threonine protein phosphatase, a novel site of action for warm-season turf, and endothall use in hybrid bermudagrass is not extensively reported. Greenhouse studies were conducted to evaluate herbicide-resistant annual bluegrass response to endothall. Five herbicide-resistant annual bluegrass biotypes were treated with increasing endothall rates and compared to two susceptible populations. One glyphosate-resistant annual bluegrass biotype was 2.3- to 3.3-fold more resistant to endothall depending on trial and susceptible biotype, and all other biotypes were endothall-susceptible. Four field studies were established from 2022-2023 to evaluate the influence of endothall rate and application timing on bermudagrass and manilagrass turf injury and annual bluegrass control. These studies were arranged as a three-by-four factorial with three levels of application timing (fully dormant, 50% green, and 100% green) and four levels of herbicide (endothall applied at 1.12, 1.68, and 2.24 kg ai ha-1 and trifloxysulfuron applied at 27.8 g ai ha-1). Maximum observed turf injury was dependent on endothall rate and timing and was commercially acceptable (<30%) at the low and middle rates when applied to 100% green bermudagrass and all rates when applied to dormant turf. When applied to 50% green turf (mid-transition), endothall unacceptably injured warm-season turf regardless of application rate, but when applied during mid-transition, only the high endothall rate unacceptably injured turf 14 days after treatment. Endothall controlled annual bluegrass more effectively when applied during mid-transition and 100% green turf than fully dormant turf. When applied at 1.68 and 2.24 kg ai ha-1 to mid-transition and 100% green turf, endothall controlled annual bluegrass 83-95%. Results from these studies indicate endothall selectively controls herbicide-resistant annual bluegrass in warm-season turf but selectivity and performance depend on application timing.