Five Most Valuable U.S. companies

Here are the five most valuable U.S. companies and their market capitalizations -- to total value of their publicly traded stock -- at the close of market Monday:

1. Apple (AAPL): $623.5 billion

2. Exxon: $405.9 billion

3. Microsoft: $257.7 billion

4. IBM: $229.1 billion

5. Wal-Mart: $244.6 billion

Source: Mercury News reporting