Cloutier v. The Queen, 2003 DTC 317 (TCC)
The taxpayer, who was a U.S. citizen and a resident of Canada, was not exempt under Article XIX on employment income earned by her as a teacher in the United States. Article XIX of the Canada-U.S. Convention was intended to deal with citizens working abroad in the discharge of a governmental function. "Functions of a governmental nature" was not intended to include functions that are commonly found in the private sector, such as education. Public school and university teaching services were intended to be governed by Article XV.
T.I. 921585 (December 1992 Access Letter, p. 24, ¶C111-048)
A Canadian citizen who is employed at the Canadian embassy in Washington will be subject to tax in Canada even after he has acquired U.S. citizenship.