The following table summarizes EMCOR’s stock option activity since December 31, 1996:

At December 31, 1999, 1998 and 1997, approximately 943,000, 642,000 and 386,000 options were exercisable, respectively. The weighted average exercise price of exercisable options at December 31, 1999, 1998 and 1997 was approximately $12.28, $8.43 and $6.46, respectively.

The following table summarizes information about EMCOR’s stock options at December 31, 1999:

The weighted average fair value of options granted during 1999, 1998 and 1997 were $18.41, $15.18 and $14.67, respectively.

The fair value of each option grant is estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions used for grants in 1999, 1998 and 1997: risk-free interest rates of 4.6% to 5.7% (representing the risk-free interest rate at the date of the grant); expected dividend yields of zero percent; expected terms of 3.3 to 6.5 years; and expected volatility of 56.5% for options granted prior to December 31, 1996, and 73.6%, 87.3% and 79.8% for options granted during 1999, 1998 and 1997, respectively.