Ah. This is clarifying:

"There are two main reasons why Zoroastrianism is not and should not be considered a dualistic religion. The first is that Zoroastrians do not consider themselves to be dualistic. In their minds, they worship one god, namely Ahura Mazda. Second, the definition of dualism lends credence to the monotheism of the Zoroastrian faith. A basic definition of dualism is “a doctrine that the universe is under the dominion of two opposing principles one of which is good and the other evil.” But Zoroastrians believe in the supremacy of Ahura Mazda."