9. The complex numbers $z_{1}, z_{2}$ satisfy $\left|z_{1}\right|=\left|z_{2}\right|=1$, and $z_{2}-z_{1}=-1$, then $\arg \frac{z_{1}}{z_{2}}=$ $\qquad$
9. $\frac{\pi}{3}$ or $\frac{5}{3} \pi$

It is easy to see that, $z_{1}=\frac{1}{2} \pm \frac{\sqrt{3}}{2} \mathrm{i}$
$$
z_{2}=-\frac{1}{2} \pm \frac{\sqrt{3}}{2} \mathrm{i}
$$

Therefore, $\arg \frac{z_{1}}{z_{2}}=\frac{\pi}{3}$ or $\frac{5}{3} \pi$