To simplify the expression \( \frac{1}{1-4} \), we start by simplifying the denominator:
\[ 1 - 4 = -3 \]
Now we can rewrite the expression as:
\[ \frac{1}{-3} \]
This can also be expressed as:
\[ -\frac{1}{3} \]
Thus, the simplified form of \( \frac{1}{1-4} \) is:
\[ -\frac{1}{3} \]