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is it normal or no?

Depends, is it a yawn or more like open-mouth breathing? Open-mouth breathing is like when a person has a stuffy nose and has to breath only through their mouth.

Is it just on the basking area, out of the water, or in the water too?

If it's open-mouth breathing, you need to get to a herp vet ASAP; he probably has a respiratory infection.

If it's just yawning, it's probably nothing, maybe he's bored, maybe he's tired.

If it's underwater, it's pretty common, like they're tasting the water. My turtle does it all the time.

it was underwater and just yawing. i think u could be right with being bored and tired. since he or she is pretty active

Underwater yawning is ok. Above the water and frequent yawning is a problem.

Always saw my turtles yawning sometimes after they climbed to basking area (tired and sleepy i think.. hehe after swimming) and sometimes underwater.. totally cute~!

My turtles will yawn both in and under water. They dont do it much though. So I wouldn't be worried.

The occasional yawn on the basking area isn't worrisome. My RES will yawn when it's evening and almost time for lights out.