Let $p>q$ be primes, such that $240 \nmid p^4-q^4$. Find the maximal value of $\frac{q} {p}$.
1. **Understanding the problem and initial conditions:**
   We are given two prime numbers \( p \) and \( q \) such that \( p > q \) and \( 240 \nmid p^4 - q^4 \). We need to find the maximal value of \( \frac{q}{p} \).

2. **Factorization and modular arithmetic:**
   Note that \( 240 = 16 \cdot 3 \cdot 5 \). We need to consider the conditions under which \( p^4 - q^4 \) is not divisible by 240. 

3. **Using Fermat's Little Theorem:**
   By Fermat's Little Theorem:
   \[
   p^4 \equiv 1 \pmod{5} \quad \text{and} \quad q^4 \equiv 1 \pmod{5}
   \]
   Therefore:
   \[
   p^4 - q^4 \equiv 0 \pmod{5}
   \]
   Similarly, since \( p \) and \( q \) are odd primes:
   \[
   p^4 \equiv 1 \pmod{3} \quad \text{and} \quad q^4 \equiv 1 \pmod{3}
   \]
   Therefore:
   \[
   p^4 - q^4 \equiv 0 \pmod{3}
   \]

4. **Considering modulo 16:**
   We need to check the divisibility by 16. Note that:
   \[
   p^4 - q^4 = (p - q)(p + q)(p^2 + q^2)
   \]
   Since \( p \) and \( q \) are odd primes, \( p \equiv 1 \pmod{2} \) and \( q \equiv 1 \pmod{2} \). Therefore:
   \[
   p^2 \equiv 1 \pmod{4} \quad \text{and} \quad q^2 \equiv 1 \pmod{4}
   \]
   Thus:
   \[
   p^2 + q^2 \equiv 2 \pmod{4}
   \]
   Also:
   \[
   p - q \equiv 0 \pmod{2} \quad \text{and} \quad p + q \equiv 2 \pmod{4}
   \]
   Hence, one of \( p - q \) or \( p + q \) is divisible by 4, and the other is divisible by 2. Therefore:
   \[
   16 \mid (p - q)(p + q)(p^2 + q^2)
   \]
   This implies \( 16 \mid p^4 - q^4 \).

5. **Conclusion from modular arithmetic:**
   Since \( p^4 - q^4 \) is divisible by 16, 3, and 5, it must be divisible by 240. This contradicts the given condition \( 240 \nmid p^4 - q^4 \). Therefore, at least one of \( p \) or \( q \) must be in the set \( \{2, 3, 5\} \).

6. **Finding the maximal value of \( \frac{q}{p} \):**
   We need to check the possible values of \( p \) and \( q \) from the set \( \{2, 3, 5\} \) and find the maximum \( \frac{q}{p} \) such that \( p > q \).

   - If \( p = 5 \), then \( q \) can be 2 or 3. The ratios are \( \frac{2}{5} = 0.4 \) and \( \frac{3}{5} = 0.6 \).
   - If \( p = 3 \), then \( q \) can be 2. The ratio is \( \frac{2}{3} \approx 0.6667 \).

   The maximum value of \( \frac{q}{p} \) is \( \frac{2}{3} \).

The final answer is \(\boxed{\frac{2}{3}}\)