A Royal Marine trainee has collapsed and died during a 30-mile march on Dartmoor.
The man, whose name is not being released, was based at the Commando Training Centre at Lympstone, Devon.
A Ministry of Defence spokesman said the cause of death was unknown.
"This incident is being investigated and the thoughts and sympathies of the Naval Service are with the family and friends of this man," he said.
The trainee died on Thursday during the so-called "30-miler" march - the last major test undertaken as part of the 32-week Commando course.
Those taking part have eight hours to complete the gruelling test while carrying additional safety equipment.
In July 2013, army reservists Edward Maher, Craig Roberts and James Dunsby died after collapsing during an SAS training exercise in the Brecon Beacons in Wales.
A number of other soldiers taking part in the programme that day, when temperatures hit 29C (84F), also collapsed and needed medical attention.
An inquest into the deaths opens later today.