President Donald Trump's eldest son and grandson took a hunting trip to Mongolia last year that reportedly cost US taxpayers over $75,000 (£59,000).
Donald Trump Jr shot a rare mountain sheep and met the country's president, according to the Citizens for Responsibility and Ethics in Washington (Crew).
Most of the money went to their Secret Service protection, the group said.
Researchers say the Trump family take 12 times as many trips as the Obamas.
The Crew report accuses the Trump family of draining the Secret Services' finances with their average of 1,000 more trips per year, many for leisure, than the previous first family.
"This trip is just one example of the expenses the Trump family is incurring with American taxpayers, and if just one of Don Jr's trophy hunting trips cost more than $75,000, it's staggering to think how high the total bill must be," the report's authors conclude.
An employee of Mr Trump Jr told CNN that the eight-day trip last August was privately paid for, with the exception of his security detail.
Mr Trump Jr is permitted, but not required, to use the Secret Service for his protection.
"As a son of the president, Donald Trump Jr is entitled to Secret Service protection and should be protected," the Crew report noted.
"But taxpayers deserve to know how much they are paying to facilitate his trophy hunting and interactions with major political donors and foreign leaders."
According to a ProPublica investigation released late last year, Mr Trump Jr killed a rare Argali mountain sheep and only retroactively secured hunting permission from the Mongolian government. ProPublica also found evidence that he was accompanied on the hunt by a Republican donor who works in the oil and gas industry.
The Argali sheep, famous for its long horns, is considered near-threatened, according to the Red List of Threatened Species.
Mr Trump Jr also reportedly met Mongolian President Khaltmaagiin Battulga, roughly one month after the president visited his father, President Trump, at the White House.
During the trip, Mr Trump Jr posted photos of himself on horseback, and him and his son outside a traditional yurt dwelling, and praised the country's "incredibly pristine land".
Crew says they determined the exact cost of the trip was $76,859.36 after two Freedom of Information Act requests were submitted to the Secret Service.
The first document provided to the group claimed that only around $17,000 was spent, Crew says, calling it "an undercount" that did not include flight costs or Mr Trump Jr's meeting in Ulaanbaatar with the president.
The second release added nearly $60,000 to the cost to taxpayers, the group found, adding that they are still investigating other aspects of the trip such as whether the US State Department was involved or whether the US Department of Interior granted a permit to bring the sheep carcass back to the US.
Unlike his sister Ivanka Trump, Mr Trump Jr holds no official government position.
He has faced criticism for his defence of gun rights and the images he posts on social media of trophy hunting trips around the world.