A CIR test has already confirmed the results.
The California State Athletic Commission will not disclose the substance that Fitch tested positive for, though multiple sources believe it to be elevated testosterone.
WSOF 16 took place on Dec. 13, and featured a welterweight title fight between Fitch and promotion champ, Rousimar Palhares.
Jon Fitch is just another name in a long list of fighters now getting caught for violating the rule book.
Fitch has the right to appeal.