Sometimes, in his off hours, Yie Eun-woong does a bit of investigative work.
He uses the Internet and other means to track personal data and home addresses of foreign English teachers across South Korea.
Then he follows them, often for weeks at a time, staking out their apartments, taking notes on their contacts and habits.
He wants to know whether they're doing drugs or molesting children.
Yie, a slender 40-year-old who owns a temporary employment agency, says he is only attempting to weed out troublemakers who have no business teaching students in South Korea, or anywhere else.
The volunteer manager of a controversial group known as the Anti-English Spectrum, Yie investigates complaints by South Korean parents, often teaming up with authorities, and turns over information from his efforts for possible prosecution.
Outraged teachers groups call Yie an instigator and a stalker.
Yie waves off the criticism. "It's not stalking, it's following," he said. "There's no law against that."
Since its founding in 2005, critics say, Yie's group has waged an invective-filled nationalistic campaign against the 20,000 foreign-born English teachers in South Korea.
On their website and through fliers, members have spread rumors of a foreign English teacher crime wave. They have alleged that some teachers are knowingly spreading AIDS, speculation that has been reported in the Korean press.
Teacher activists acknowledge that a few foreign English instructors are arrested each year in South Korea -- cases mostly involving the use of marijuana -- but they insist that the rate of such incidents is far lower than for the Korean population itself.
"Why are they following teachers? That's a job for the police," said Dann Gaymer, a spokesman for the Assn. for Teachers of English in Korea. "What this group is up to is something called vigilantism, and I don't like the sound of that."
In November, the president of the teachers group received anonymous e-mails threatening his life and accusing him of committing sex crimes.
"I have organized the KEK (Kill White in Korea)," one e-mail read in part. "We will start to kill and hit [foreigners] from this Christmas. Don't make a fuss. . . . Just get out."
Yie acknowledges that he has been questioned by investigators but denies any involvement in the threats of violence.
"To be honest," he said, "a lot of our group members believe the teachers made this all up."
The debate over foreign English teachers is symbolic of a social shift taking place in a nation that has long prided itself on its racial purity and singular culture, South Korean analysts say.
In less than a decade, the number of foreigners living in South Korea, with a population of nearly 49 million, has doubled to 1.2 million, many of them migrant workers from other Asian nations.
Also included are the foreign English teachers, most from the United States, drawn here by compensation packages that may include as much as $2,500 a month plus free rent and a round-trip ticket to teach a Korean population obsessed with learning from native speakers.
Yie's efforts have the support of some educators who say many foreign teachers lack the skills to run a classroom.
"This has nothing to do with race. It is all about teaching," said Kim Young-Lan, a sociology professor at Sookmyung Women's University in Seoul.
The government has tried to stem what it sees as a troubling number of racist incidents. A 31-year-old man was charged last year for a verbal outburst against an Indian man and a Korean woman traveling together on a city bus in Seoul.
But some teachers from abroad say Korean laws regarding their status remain discriminatory. Foreign English teachers must undergo HIV tests and criminal and academic checks that are not required of Koreans doing the same work, they say.
Yie says he has nothing against foreigners. Growing up near the city of Osan, he often rode with his taxi driver father and encountered foreigners who served at the U.S. military base there. "I learned to pick out the good guys from the bad guys," he says
In 2005, by then living in Seoul, he joined the fledgling activist group after seeing an upsetting posting on a website: claims by foreign teachers that they had slept with Korean students.
Yie, who is single and has no children, volunteered to help organize an effort to rein in such behavior.
"People were angry; most of them were parents with kids," he said. "We all got together online and traded information."
Gaymer says he doubts that such a posting ever existed. Instead, he says, Koreans were angry about photos posted on a job website showing foreigners dancing with scantily clad Korean women.
"They were consenting adults at a party with foreign men," he said. "They weren't doing anything bad or illegal."
Yie's group, Gaymer says, has used the incident as a rallying call. "They're posting online pictures of teachers' apartments and whipping each other into a nationalist frenzy, creating a hysteria against all English teachers, troublemakers or not," he said.
Yie, who says his group is managed by half a dozen key figures and has 300 other members, created a system for parents and others to report bad teachers. The group says it has contributed to several arrests, including the recent bust of several foreign instructors for gambling and marijuana possession.
"I'm being called a racist who judges the entire group by the mistakes of the few," Yie said. "I'm trying to look at these teachers with an open mind."
Ju-min Park of The Times' Seoul Bureau contributed to this report.