Poachers have killed nearly half of Mozambique's elephants for their ivory in the past five years, says the Wildlife Conservation Society (WCS).
The campaign group estimates there has been a 48% decline in elephant numbers from just over 20,000 to around 10,300.
The WCS's Alastair Nelson says the poachers come from Tanzania where the elephant population has already been decimated.
China is the world's largest consumer of smuggled tusks.
Remote northern Mozambique, which includes the Niassa National Reserve, was the hardest hit, accounting for 95% of elephant deaths.
The north "has always been a remote and poorly governed area, with an underlying level of corruption," Mr Nelson told AFP.
Mozambique did not criminalise the killing of protected animals until 2014.
In May, police seized the country's biggest ever find of illegal wildlife products.
Some 1.3 tonnes of elephant ivory and rhino horn were found. That find represented the result of the killing about 200 animals and had a street value of $6.3m (£4.1m).