Quoted from my 401k plan website:
How is the unit price calculated?
* The number of "units" in your account determines your ownership in the fund. The number of units in your account can increase or decrease, depending on the account activity. For example, contributions to the fund increase the number of units, and payments from the fund decrease the number.
* Determine the market value of your investment in the fund by multiplying the number of units in your account by the net asset value (NAV): Units x NAV = Market Value of Your Investment in the Fund
* A NAV is calculated each business day the New York Stock Exchange (NYSE) is open. The NAV includes gains and losses in the underlying assets of the fund and also can include factors such as plan expenses or interest accruals. The NAV is determined using the NYSE closing price.
What’s the company stock fund?
The company stock fund is valued using unit accounting. When you invest in the company stock fund, you don't directly own shares of company stock. Instead, you own a portion of the company stock fund, which is made up of company common stock and a small amount of cash that provides liquidity for transactions.
I use MS Money to track all my investments. For my 401k, I manually update the price each paycheck when I add more money into it. I would love it if it had a ticker symbol that updated automatically, but that ain't going to happen according to the plan.