The US population has hit 300 million people, just 39 years after it reached 200 million, according to US Census Bureau estimates.
The population reached the milestone at 0746 (1146GMT) - a timing based on calculations that factor in birth and death rates and migration.
The bureau's maths suggests that the US gains one person every 11 seconds.
But it is not possible to say if the 300-millionth American was a new-born or crossed one of the US borders.
Correspondents say that there is not expected to be the same hullabaloo as when the figure of 100 million was reached in 1915, or the double century in 1967 when President Johnson gave a speech and newborn Robert Ken Woo Jr was hailed the 200-millionth American by Life magazine.
Today, the population figure is mired in the divisive politics of immigration - a hot-button issue ahead of the 7 November mid-term elections, they say.
The population in the US is the third largest in the world, behind China and India.
According to the Census Bureau, 14% of the current US population is Hispanic, compared to 4% in 1966, and it is projected that a quarter of the population will be Hispanic in 2050.
It is also expected that in the next 50 years there will be more Hispanic births in the US than immigrants.
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Michael Replogle, of Environmental Defense, told the Associated Press news agency: "If the population grows in thriving existing communities, restoring the historic density of older communities, we can easily sustain that growth and create a more efficient economy without sacrificing the environment."
Vicky Markham, director of the Center for Environment and Population, said "sprawl has become the most predominant form of land use", with the US becoming a "suburban nation".
"Sprawl is, by definition, more spread out. That of course requires more vehicles and more vehicle miles travelled," she told AP.
Other figures released by the Census Bureau, show how America has been changing since previous population milestones.
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