(Bloomberg News) Barton Biggs, who increased bets on U.S. equities before the Standard & Poor's 500 Index rallied last month, said he remains bullish even amid concerns over progress in solving Europe's debt crisis.

"I'm terrified I'm not long enough if we're going to have a strong rally here, which we could," he said during an interview on Bloomberg Television's "In the Loop" with Betty Liu today. Biggs said his net-long position in equities is 65 percent. At the same time, "I'm terrified I'm too long if the apocalypse is coming in Europe," he said.

The founder of Traxis Partners LP said on Dec. 12 that he was investing in U.S. and Asian stocks. Biggs said at the time that equities might rise or fall 20 percent because of concern about budget negotiations and Europe. The S&P 500 has increased 6.4 percent since then through Jan. 20.

His optimism on stocks has fluctuated along with the market. Biggs reduced the net-long position, a gauge of bullish versus bearish investments, in the Traxis Global Equity Macro Fund to about 40 percent at the end of September before increasing it to 65 percent on Oct. 17 and 80 percent on Oct. 31, according to interviews with Bloomberg. On Nov. 21, he said he cut the level to less than 40 percent. On Dec. 2, he said he boosted it to about 60 percent.

U.S. stocks have risen for three weeks, the longest streak since October, as better-than-estimated economic data and company earnings boosted confidence in American growth. The S&P 500 gained 4.6 percent in 2012 through last week, the best start to a year since 1997. The benchmark gauge for U.S. equities rose 0.4 percent to 1,320.82 at 9:57 a.m. New York time today.