Chinese authorities are investigating three senior executives of state-owned energy firm PetroChina over "serious breaches of discipline".
China's State-Owned Assets Supervision and Administration Commission, which oversees state firms, did not provide details of the breaches.
PetroChina said two vice-presidents and its chief geologist resigned on Tuesday because of "personal reasons".
The firm's shares fell as much as 5% in early trade in Hong Kong.
PetroChina said in a statement that Vice Presidents Li Hualin and Ran Xinquan as well as Chief Geologist Mr Wang Daofu "are currently under investigation by relevant People's Republic of China authorities".
It added that the three officials had resigned from their respective positions "with immediate effect".
On Monday, the Ministry of Supervision said that Wang Yongchun - another top official at China National Petroleum Corporation (CNPC), the parent firm of PetroChina - had been put under investigation. It did not provide any details about the probe.
In recent months Chinese authorities have been engaged in a high-profile anti-corruption campaign initiated by new President Xi Jinping, who has called corruption a threat to the survival of the ruling Communist Party.
Earlier this month, a former top economic official was expelled from the Communist Party and removed from public office.
State-owned Xinhua news agency reported that Liu Tienan, the former deputy head of the National Development and Reform Commission, had "accepted huge amounts of bribes".
In July, former Railways Minister Liu Zhijun was given a suspended death sentence for corruption and abuse of power.