alternately; in turn
彼らは交互に車を運転した。
They took turns driving the car.
我々は、交互に休息をとった。
We took turns resting.
それに、彼らが私と海底とを交互に見つめていたのは、どう見ても明らかだった。
Besides, it was obvious that they were looking back and forth between me and the seabed.