Use Conditional Formatting off the Format menu
Select/highlight the range you want to format, then use the menu to call up
the Conditional formatting dialog
Select 'Formula Is' rather than 'Cell Value Is' and enter this formula
=mod(row(),2)=0
Then click on the format button, go to the pattern tab, and choose the color
you want the rows to be shaded. Click on OK & you're done!
Duke