TOKYO, DEC. 4 -- Ever since he began his quixotic effort last year to gain membership on the board of a Japanese auto-parts company, T. Boone Pickens has portrayed himself as an earnest investor whose shareholder rights were being thwarted by Japan's insular corporate system.

But today, Pickens's campaign against Japan Inc. got a black eye when he was forced to acknowledge something his detractors have long alleged -- that his investment in the auto-parts firm, Koito Manufacturing Co., is backed by a Japanese stock speculator known for his hardball tactics.

The development is the latest twist in a long-running battle that many analysts say has highlighted the worst aspects of both the American and Japanese corporate cultures.

Pickens has been widely accused of engaging in fast-buck artistry and political showboating, while his adversaries in Koito's management have been denounced for their xenophobia and cozy ties with other Japanese firms, notably Toyota Motor Corp., a major Koito customer and shareholder.

Today's revelation came because Pickens found himself required under a new Japanese regulation to disclose the nature of his relationship with the speculator, Kitaro Watanabe.

In a telephone interview, the Texas oilman provided a summary of the disclosure forms that, he said, he will file here this week. The most noteworthy part of the disclosure sheds light on the arrangements behind Pickens's purchase from Watanabe of his 26.4 percent Koito stake, now valued at more than $1 billion.

The disclosure, according to Pickens's summary, will show that Watanabe lent Pickens the money to buy the shares and that the only collateral for the loan was the shares themselves.

That revelation provides ammunition to Pickens's critics, who scoff at his claim that he is seeking a spot on Koito's board because he wants to improve Koito's operations and maximize his return on investment.

The story according to Pickens's detractors is that the Texas oilman was recruited by Watanabe to help make life miserable for Koito's management, in the hope that Koito would arrange to have the pair bought out at a tidy profit. Watanabe allegedly has a history of employing such tactics and had previously tried -- unsuccessfully -- to persuade Toyota to buy his Koito stake, according to Koito officials.

Today's disclosure is embarrassing for Pickens because it lends credence to those who say he has been acting on Watanabe's behalf. The information suggests that Watanabe offered Pickens a low-risk proposition to enlist his assistance: a loan the oilman could simply walk away from if he couldn't make money on his investment.

Today's disclosure notwithstanding, even Japanese observers who have harshly attacked Pickens acknowledge that his campaign has raised several valid criticisms of Japanese corporate practices. The Texan has drawn grudging admiration here for championing the cause of small shareholders, who are generally shown little regard by Japanese executives.

Pickens has also given visibility to the U.S. government's complaint that Japan's corporate families, known as keiretsu, tend to band together to keep foreign goods and investors from penetrating the Japanese market.

But the new information seems sure to damage Pickens's credibility. In the interview, Pickens sought to play down the importance of the fact that he borrowed on such easy terms. "I certainly don't take the {loan} lightly," he said, adding, "I have never defaulted" on any loan.