In the pantheon of Wall Street superstars, 44-year-old Bill Ackman is a relative new kid on the block. But in a short time, the founder of Pershing Square Capital Management has staked out a reputation as a powerful "activist investor."

Which is, essentially, a more polite term for "corporate raider," entailing buying up common stocks in public companies and pushing for changes, so the market can realize their value.

Through Pershing, Ackman has acquired controlling positions in many large corporations including Wendy's, McDonald's, and Borders Group (NYSE: BGP).

Despite a somewhat embarrassing failure with Target (NYSE: TGT) that cost investors somewhere in the ballpark of $2 billion, Ackman's reputation emerged relatively unscathed -- he recently made Portfolio.com's list of up-and-coming Wall Street wise men. And last year, Ackman saw a 30 percent gain in his investments, putting him among the industry's top performers.

So what might you find in the young shareholder activist's portfolio? Thanks to the SEC's 13F filing regulation, requiring institutional investment managers overseeing more than $100 million to submit quarterly reports of their holdings, we can see how Ackman was positioning his assets as of the end of last year.

Form 13F shows that Ackman reduced his holdings in Kraft Foods (NYSE: KFT) and Target, while adding shares of General Motors (NYSE: GM). J.C. Penney (NYSE: JCP) now ranks as Ackman's most valuable position, following his acquisition of 34.8 million shares in the retailer. In so doing, Ackman landed a seat on the company's board of directors and is currently using his power to push for major changes.

Pershing also continues to hold its large stake in Citigroup (NYSE: C), a major purchase that made financial headlines last year. (Click here to access free, interactive tools to analyze all the stocks mentioned below.)

Here are some of Ackman's top holdings for the last quarter of 2010:

Company

Shares Held-09/30/2010

Shares Held-12/31/2010

% Change in Shares Held

% of Portfolio

J.C. Penney

15,711,994.00

34,919,071.00

122.24%

19.78%

General Growth Properties (NYSE: GGP)

23,953,782.00

69,965,786.00

192.09%

18.99%

Fortune Brands (NYSE: FO)

7,465,555.00

16,668,636.00

123.27%

17.61%

Citigroup

146,475,426.00

146,514,287.00

.03%

12.15%

Kraft Foods

27,632,023.00

19,470,089.00

-29.54%

10.76%

Target

14,143,896.00

7,399,622.00

-47.68%

7.8%

General Motors

0.00

7,187,740.00

New Positions

4.65%

Corrections Corporation of America (NYSE: CXW)

10,936,672.00

7,827,430.00

-28.43%

3.44%

Howard Hughes (NYSE: HHC)

0.00

3,568,017.00

New Positions

3.4%

Automatic Data Processing (NYSE: ADP)

8,700,612.00

1,404,733.00

-83.85%

1.14%

Borders Group

10,597,980.00

10,597,980.00

0.00%

0.17%

Greenlight Capital Re (Nasdaq: GLRE)

250,000.00

250,000.00

0.00%

0.12%

Interactive Chart: Press Play to compare changes in analyst ratings over the last two years for the top six stocks mentioned above. Analyst ratings sourced from Zacks Investment Research. Note: The numbers on top of items represent the forward P/E ratio, if available.


Kapitall's Eben Esterhuizen and Alicia Sellitti do not own shares of any companies mentioned.

General Motors is a Motley Fool Inside Value selection. Corrections Corporation of America and Fortune Brands are Motley Fool Stock Advisor picks. Automatic Data Processing is a Motley Fool Income Investor recommendation. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.