A fluid mixing system includes a container, such as a flexible bag, bounding a compartment. A flexible drive line is disposed within the compartment, the drive line having a first end rotatably connected to a first end of the container and an opposing second end rotatably connected to a second end of the container. At least one mixing element, such as an impeller, can be coupled with the flexible drive line. Rotation of the drive line facilitates rotation of the impeller within the container.