Let’s say you have a 401(k) with a balance of $10k and also taxable assets of $10k, for $20,000 total. You choose to have $10k in stocks, $5k in bonds, and $5k in cash for your emergency account. The “traditional” placement for an emergency fund is in your regular taxable account, perhaps in a bank savings account. The rest of the assets are distributed according to this tax-efficient placement chart.

However, in this scenario all your interest earned on your cash will be taxed at your marginal ordinary income tax rate, which can be as high as 35%. See table of 2009 Marginal Tax Rate Brackets. Meanwhile, your stocks will mostly give off dividends, which are taxed at a current maximum rate of 15%, and possibly quite less. So why not put the cash into the 401k?