Pershing Square USA had a tough first day of trading, falling double digits during Wednesday's session. The IPO marked Bill Ackman's public trading trading debut. The hedge fund legend has been vocal about his aspirations to operate a fund like Warren Buffett 's Berkshire Hathaway. Pershing Square USA, the firm's close-end fund, trades under the ticker PSUS while Pershing Square, the firm's asset management arm, is listed as PS. PSUS stock closed at $40.93, down 18% from its IPO price of $50.