New York hedge fund manager Eric Rosenfeld has been classed among the nation’s “wolf pack” of activist shareholders. Now Rosenfeld and his firm, Crescendo Partners, seem to have set their sights on Build-A-Bear Workshop.

Crescendo has invested more than $2.4 million to increase its holdings in the stuffed-toy retailer over the past month, buying more than 415,600 shares since July 6, according to a recent filing with the Securities and Exchange Commission. Crescendo and Rosenfeld now hold a 7.6 percent stake in Build-A-Bear.

“We think the stock is undervalued. That’s why we’re buying,” Rosenfeld said, declining to comment on any plans his firm may have for its involvement with Build-A-Bear. Build-A-Bear’s stock closed Aug. 11 at $5.72, down 41.4 percent from its 52-week high of $9.76.

However, Rosenfeld is not known for sitting quietly on the sidelines. Crescendo Partners, and Rosenfeld specifically, were included in a list of “The Wolf Pack” activists as part of a recent report on shareholder activism prepared by Mark Harnett of MacKenzie Partners, a New York-based mergers and acquisitions consulting firm.

Rosenfeld has been involved in a variety of proxy fights over the years with companies ranging from Canadian IT firms to U.S. women’s apparel retailers. In 2007, Crescendo launched an unsuccessful attempt to prevent baseball card company Topps Co. from being acquired by former Walt Disney Chief Executive Michael Eisner and Madison Dearborn Partners in a $9.75 million merger. The previous year, Crescendo had teamed up with Pembridge Capital Management of New York, under the guise of the Topps Full Value Committee, to launch a proxy fight to add three directors of their selection to Topps’ board. The committee was critical of Topps’ executive compensation, its acquisition record and its failure to maximize the value of the Bazooka bubblegum brand.

In 2008, Crescendo launched a proxy fight seeking greater board control of Charming Shoppes Inc., the Bensalem, Pa.-based retailer that owns the Lane Bryant and Fashion Bug brands. That March, Charming Shoppes filed a lawsuit against Crescendo and the New York-based Myca Partners hedge fund, which were operating jointly as the Charming Shoppes Full Value Committee. Charming’s suit claimed the funds had violated SEC regulations by filing misleading and incomplete documents about the company.

Charming Shoppes stated in its complaint that the hedge funds had “depicted themselves to the investing public as legitimate investors and ‘would-be’ directors. ... At no time have defendants disclosed any part of their true track record of using proxy fights to disrupt corporations and to profit by forcing them to sell assets, buy back stock or buy off defendants and their cronies.” In May 2008, Charming Shoppes and the hedge funds ended the battle via an agreement that created a new board made up of two members of Charming Shoppes’ management, two nominees from the hedge funds and two independent retail executives.

Crescendo has pushed for similar board representation with companies such as Nashville, Tenn.-based restaurant chain O’Charley’s Inc., Ottawa-based tech firm Bridgewater Systems, Canadian sporting goods retailer Forzani Group, and other Canadian firms, including Call-Net Enterprises Inc., Geac Computer Corp. Ltd., Spar Aerospace and Dalsa Corp.

Crescendo recently increased its stake in Regis Corp., which owns the Hair Club for Men and the Supercuts beauty salon chain, and delivered a letter to the company in late July nominating candidates for Regis’ board. On Aug. 5, Regis announced it was exploring strategic alternatives, which could include a potential sale, a move applauded by Crescendo and Rosenfeld.

Build-A-Bear also considered strategic alternatives for the company in 2007. In March 2008, it said it concluded the process and decided to double its share repurchase program to up to $50 million.

Shareholder activism is on the rise nationwide, prompted in part by increasing pressure on companies to improve shareholder return and a loss of confidence in existing company boards and management. According to a report from Boston-based law firm Goodwin Procter, the number of proxy fights launched through October 2009 was 132, up nearly 14 percent from the same period in 2007. Dissident shareholders were successful in obtaining at least one board seat in more than 70 percent of the proxy battles, according to the report.

Crescendo also currently holds a stake in Charming Shoppes, O’Charley’s, Destination Maternity Corp., Canadian beverage maker Cott Corp., Canadian gold company Crystallex International Corp., and New York-based aircraft parts firm CPI Aerostructures Inc., according to a recent SEC filing.

Crescendo is Build-A-Bear’s fourth-largest shareholder. Chairman and Chief Executive Maxine Clark is the largest shareholder, with a 12.9 percent stake, followed by Indiana-based BML Investment Partners with a 10.9 percent share and New York-based Paradigm Capital Management Inc. with a 9.7 percent share, according to Build-A-Bear’s most recent proxy filing.

Prior to its recent stock purchases, Crescendo held a 5.6 percent stake in the company. Rosenfeld said the firm has been a Build-A-Bear investor for “quite some time.”

Build-A-Bear reported $74.1 million in second quarter revenue, down 10.5 percent from the second quarter of 2009. The company reported a loss for the quarter of $8.5 million, compared with $6 million in the same time period last year.

Build-A-Bear operates 346 company-owned stores. The company did not return calls for comment.