Answer
$$\frac{a^{4}}{b^{5}}$$
Work Step by Step
To divide powers of the same base, subtract the exponents:
$$\frac{a^{5}b^{3}}{ab^{8}} =a^{5-1}b^{3-8}=a^{4}b^{-5}.$$
A negative power is equivalent to the reciprocal with the positive power:
$$a^{4}b^{-5}= \frac{a^{4}}{b^{5}}.$$