If you are using multiple Oracle Homes, it is most likely that PL/SQL Developer is using a different Oracle Home than SQL*Plus. By default, PL/SQL Developer will use the Primary Oracle Home (the one that is listed first in the PATH environment variable.

There are 3 ways to resolve this:<OL TYPE=1>
[*] Start PL/SQL Developer, go to Tools > Preferences > Options tab page, and select the correct Oracle Home from the list. Restart PL/SQL Developer and try to connect again.

[*] Use the Oracle Home Selector (a standard Oracle utility) to select a different Primary Oracle Home.

[*] Add the unknown alias through the Oracle Net Configuration Assistant.
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