Zoltek Cos. Inc. fired its accounting firm, PricewaterhouseCoopers LLP, and hired Grant Thornton LLP as its new independent registered public accounting firm, according to a regulatory filing Friday.

Grant Thornton's hiring was effective May 21.

Zoltek's Audit Committee "participated in and approved the decision to change its independent registered public accounting firm," according to the firm's Securities and Exchange Commission filing.

Zoltek officials said in the filing that between September 2005 and May 21 "there have been no disagreements with PricewaterhouseCoopers LLP on any matter of accounting principles or practices." Zoltek went on to say that it incurred weaknesses in internal control over financial reporting from September 2004 to May 21.

Zoltek also said it requested that PricewaterhouseCoopers furnish the company with a letter addressed to the SEC stating whether or not it agrees with Zoltek's statements.

St. Louis-based Zoltek Cos. Inc. (Nasdaq: ZOLT) manufactures carbon fiber products.