The Pioneer Oak Ridge Small-Cap Growth Fund/A (ORIGX), a top performing fund that invests in small companies, will close to most new investors on Wednesday.

The action is aimed at keeping assets in the fund from increasing to the point where they hurt its performance.

Pioneer has planned to close the fund when its assets reach about $225 million, the fund company said in a regulatory filing. The fund will remain open to new investors who buy shares for retirement accounts, such as 401(k) plans.

For the three-year period ended in April, Pioneer Oak Ridge Small-Cap Growth Fund registered an average annualized return of 8.4%, versus a gain of 2.1% for its peers. For the ten-year period ended last month, the fund gained 11.9% on average, versus 8.2% for the average small-cap growth fund.

Pioneer Oak Ridge Small-Cap Growth is ranked 5 Stars by Standard & Poor’s.

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