His big weighting in the company is also helping his performance, which may in turn bring in more investor dollars. Burnham's fund, which also has significant positions in Chipotle Mexican Grill and Williams Companies Inc, is up 4.8 percent for the year to date, according to Lipper, a return about 4 percentage points better than the S&P 500.

Over the last 5 years, the fund has returned an average of 14.3 percent a year, a performance slightly better than average large cap fund. The fund costs $1.36 per $100 invested, a rate slightly above average.

Other fund managers with large stakes in Apple who aren't selling say that they didn't set out to have an oversized position in the company.

David Chiueh, the manager of the Upright Growth fund, has 13.4 percent in Apple, the second-largest among diversified funds tracked by Lipper, mostly because shares he bought in 2008 have appreciated, he said. The BlackRock Science and Technology Opportunities Portfolio, the third-largest Apple holder, has an underweight position according to the fund's chosen benchmark, the MSCI World Information Technology index, a company spokeswoman said.

Other large holders of Apple have started to trim their positions. Mark Mulholland, whose Matthew 25 fund is classified as a non-diversified fund, has 15.3 percent of his portfolio's assets in Apple.

He expects to trim the position down to 10 percent of his portfolio, in part because the company's shares do not look as attractive on a valuation basis, he said.

"It's not a company under duress by any means, but it's not trading at as big as a discount as it was before," Mulholland said.

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