[jeremyrparker]

I need my mothers maiden name for leagl purposes. I was adopted

[tickle]

If you are adopted, your legal last name is your adopted parents name.

[JudyKayTee]

i need my mothers maiden name for leagl purposes. i was adopted

If you are legally adopted your mother's maiden name is your adopted mother's maiden name.

[MsMewiththat]

Absolutley correct, the maiden name is the maiden name of your adopted mother.
If in fact you need the birth mother's maiden name, that is assuming you know that she is maried. Can I ask what the legal purpose would be for asking this question? There is a way of going about finding out medical history without disclosing their identity, you can also do a search to find their identity if they are interested in being located. Please write back and let me know, if your comfortable, what that need might be.
Thanks in advance.

[Fr_Chuck]

No, there is no legal reason you even need to know your bio parents name. Your mother is the lady who adopted you, and it is her maiden name that is your mothers maiden name