If you have a fly speed, can you 5-foot step directly up?
From the Fly skill:
Without making a check, a flying creature can remain flying at the end of its turn so long as it moves a distance greater than half its speed. It can also turn up to 45° by sacrificing 5 feet of movement, can rise at half speed at an angle of 45°…
Fly: When a character or creature is flying, and that creature decides to ascend at a 45° angle, the rules states that it moves at half speed? Do the rules for diagonal square counting still apply when moving up diagonally in this way?
No. Since the game is generally assumed to be played in two dimensions, even when representing three dimensional combat, the rules for ascending are handled by the speed reduction instead of asking players and GMs to ascertain the diagonal vertical movement.
Does this negate your ability to '5-foot step' vertically, or could you still move diagonally one square?
Best Answer
No.
You cannot take a 5-foot step upwards because doing so would take at least 10 feet of movement, which invalidates the 5-foot-step action and becomes a Move Action instead.
Anytime you cannot spend only 5-foot of your movement to enter a square, it should require a move action, and thus it is no longer a 5-foot step.
If you moved 10-foot, it is no longer a 5-foot step. This has been discussed on paizo's messageboards here and here.
According to James Jacobs (Pathfinder's Creative Director), a creature should not be able to fly upwards as a 5-foot step.
The only exception for this, back in 3.5, was creatures that had Perfect maneuvering, which allowed them to move upwards without using double their movement, so they could move 5 feet upwards without using 10 feet of their movement. This exception no longer exists in Pathfinder, but if a creature has a similar ability, the exception should apply.