Matthew Rhys only speaks Welsh to his baby son.
Keri Russell and Matthew Rhys
The 'Americans' actor and partner Keri Russell welcomed baby Sam into the world last May and the 42-year-old star is determined the tot will pick up his native language.
Asked if he will teach Sam to speak Welsh, he said: "I am speaking purely in Welsh to him, that's it.
"His replies, you'd think he was speaking in Welsh as well. It's quite a gutteral language."
Keri also has son River, nine, and daughter Willa, four, from her marriage to Shane Deary, and Matthew says they have adapted well to having a baby brother.
Speaking on 'Live! With Kelly', he said: "The older ones are brilliant, really good, they've claimed him as their own."
But the first-time father admitted he worries a lot about the baby, even though Keri always tries to put him at ease.
He said: "She's been a reassuring hand. I'm the one going, 'What about...' and she goes, 'Shut up it's fine.'
"There was a moment recently when - I said, 'He has a very heavy cold and we've got to take him to the doctor.' She's like, 'He has a cold, I'm gonna get my hair blown out.' I go, 'I'm gonna take him to the doctor.' She went, 'You take him to the doctor and when the doctor tells you he has a light cold, text me.' "
Asked whether he then texted her three minutes later, he added: "Probably less. It was more like two. It's a cold, we're fine!"
Keri and Matthew - who play married undercover KGB agents in 'The Americans' - have been extremely private about their romance and only made their first public appearance as a couple in April 2014 after dating for some time.
Tagged in Keri Russell Matthew Rhys