Investors in a hedge fund run by James Simons are questioning why a fund held by the mathematics professor-turned-investor and his colleagues has made big gains while another held mostly by outside investors has lost money, The Wall Street Journal reported.

Citing an investors' conference call, the paper said the investors asked Mr. Simons about why his Renaissance Institutional Equities Fund (RIEF) lost 17% this year through April, while another fund held nearly exclusively by Mr. Simons and his colleagues earned a 12% return.

Renaissance Technologies LLC declined to comment.

The Journal said during the conference call Mr. Simons had reiterated his views from a monthly investor letter where he said that RIEF suffered a "performance onslaught" during an "extreme market rally."

"We certainly understand our clients' discomfort," Mr. Simons was quoted as writing in the letter.

RIEF lost money in 2008, while the internal fund jumped 80 percent, the paper said.

Renaissance does not employ outside administrators, something many investors are demanding following the Madoff scandal.