NEW YORK (TheStreet) -- Actively managed mutual funds traditionally earned their fees by picking the most attractive stocks and bonds. But these days more portfolio managers are investing in ETFs.

According to Morningstar, hundreds of mutual funds have taken the plunge. More than 130 active mutual funds own

SPDR S&P 500

(SPY) - Get Report

. Investors in the S&P 500 ETF include

Columbia Dividend Income

(LBSAX) - Get Report

,

Hartford Growth Opportunities

(HGOAX) - Get Report

and

AllianceBernstein Dynamic All Market

(ADAAX). More than 80 funds have invested in

iShares MSCI EAFE

(EFA) - Get Report

, including

PIMCO Global Multi-Asset

(PGMAX) - Get Report

,

Sterling Capital Strategic Allocation Equity

(BCAAX)

and

USAA Global Opportunities

(UGOFX) - Get Report

.

Not so long ago, active mutual funds stayed clear of ETFs. After all, active managers were supposed to outdo benchmarks -- not load up on index funds. But gradually portfolio managers have discovered that ETFs can play useful roles. "ETFs can be convenient ways to gain market exposure quickly," says Cory Banks, managing editor of ETF Report.

Portfolio managers use ETFs in a variety of ways. While some managers take big positions in ETFs, many funds have only limited stakes.

Vanguard Windsor

(VWNDX) - Get Report

, an active large value fund, has 0.9% of its assets in

Vanguard Value ETF

(VTV) - Get Report

, and

Vanguard Morgan Growth

(VMRGX)

has 0.8% in

Vanguard Growth

(VUG) - Get Report

.

For such active funds, ETFs can offer easy ways to manage cash. Say investors suddenly pour money into a fund, and the portfolio manager can't decide which stocks to buy. To avoid sitting in cash -- which could be a drag on returns -- the manager could hold an ETF as a temporary measure.

ETFs can also be used to manage taxes. Say a manager has a loss in a stock but doesn't want to give up on the holding. The fund can sell the shares, book the loss, and put the proceeds in cash. Under IRS rules, the manager must wait more than 30 days to buy back the shares. Instead of keeping cash, the fund may decide to hold an ETF for a few weeks and then repurchase the original stock.

ETFs have become popular with asset allocation funds. These attempt to outdo the benchmarks by shifting sector allocations. By using ETFs, portfolio managers can quickly target asset classes. AllianceBernstein Dynamic All Market uses a mix that includes futures and ETFs. The fund recently had 20% of assets in SPDR S&P 500 and 5% in

Vanguard MSCI Emerging Markets ETF

(VWO) - Get Report

.

RiverFront Global Allocation

(RMGAX)

holds a mix of stocks and ETFs. The portfolio has 8.5% of assets in

Vanguard Large-Cap ETF

(VV) - Get Report

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, 6.0% in

iShares S&P SmallCap 600 Index

(IJR) - Get Report

and 5.6% in

SPDR Barclays Capital High Yield Bond

(JNK) - Get Report

.

Many funds have turned to ETFs as a way to hold gold. A popular choice is

SPDR Gold Shares

(GLD) - Get Report

, which owns bullion. As an insurance policy,

First Eagle Fund of America

(FEFAX) - Get Report

has 3.4% of assets in the ETF.

Midas Perpetual Portfolio

(MPERX)

, which aims to protect assets in all market conditions, typically keeps about 20% of assets in gold and 10% in silver. Along with mining stocks, the fund currently has 18.5% of assets in SPDR Gold Shares and 4.3% in

iShares Silver

(SLV) - Get Report

.

A growing number of mutual funds keep nearly all their stock assets in ETFs.

Stadion Managed Portfolio

(ETFFX)

relies on a variety of momentum indicators to pick holdings. If technology stocks are climbing, the fund buys an ETF in that sector. With large stocks climbing last summer, the fund had 25% of assets in SPDR S&P 500, 20% in

iShares S&P 100 Index

(OEF) - Get Report

and 20% in

SPDR Dow Jones Industrial Average

(DIA) - Get Report

.

Some retirement funds rely heavily on ETFs. Managed by

Lincoln Financial Group

,

Presidential Protected Profile 2010

(PZAAX)

is a cautious choice that emphasizes bond ETFs. The fund has 12% of assets in

iShares Barclays TIPS Bond

(TIP) - Get Report

and 24% in

Vanguard Total Bond Market ETF

(BND) - Get Report

.

Presidential Protected Profile 2050

(PZEAX)

, a more aggressive choice for young people, has 25% of assets in Vanguard MSCI EAFE ETF and 5.7% in

iShares Russell 2000 Index

(IWM) - Get Report

.

By focusing on ETFs, the funds can hold down fees, an important consideration at a time when many retirees are keeping a close eye on costs.

This article is commentary by an independent contributor, separate from TheStreet's regular news coverage.

Stan Luxenberg is a freelance writer specializing in mutual funds and investing. He was executive editor of Individual Investor magazine.