A quick google search will give you more information, here is one of the first links that come up :
http://www.isla-offshore.com/going-offshore/cgt-exemption-for-foreign-trader/
"Capital Gains Tax (CGT) Exemption in the U.S.
Under the general rule capital gains of non-residents received from U.S. sources are not taxed with the U.S. This rule should also apply to Forex ordinary income with IRC 988 and short-term capital gains from securities."
If you look a little further you will find the IRS links
http://www.isla-offshore.com/going-offshore/cgt-exemption-for-foreign-trader/
"Capital Gains Tax (CGT) Exemption in the U.S.
Under the general rule capital gains of non-residents received from U.S. sources are not taxed with the U.S. This rule should also apply to Forex ordinary income with IRC 988 and short-term capital gains from securities."
If you look a little further you will find the IRS links
from U.S. Sources.