Let's multiply:

\( 123456789123456789 \times 987654321987654321 \)

Calculating exactly:

\[
123456789123456789 \times 987654321987654321 = 121932631356500531347203169112635269
\]

**Final Answer:**
\[
\boxed{121932631356500531347203169112635269}
\]
