There are two serial pragmas, and both apply to "for" loops:
#pragma MP serial_loop
#pragma MP serial_loop_nested
The #pragma MP serial_loop pragma indicates to the compiler that the next for loop is not to be implicitly/automatically parallelized.
The #pragma MP serial_loop_nested pragma indicates to the compiler that the next for loop and any for loops nested within the scope of this for loop are not to be implicitly/automatically parallelized. The scope of the serial_loop_nested pragma does not extend beyond the scope of the loop to which it applies.