Two workers died while three others fell sick after inhaling toxic gas inside a septic tank in the East Ilisha area of Bhola's Sadar upazila on Saturday morning.
The deceased have been identified as Abdul Malek, 45, and Jashim, 40.
Local people said the five workers had only built the septic tank in the house of Nirob Bulai in the Ponditer Pole area of East Ilisha a month ago.
On Saturday morning, Abdul went to Nirob's house to inspect the newly built septic tank.
Abdul first got down into the septic tank. When he failed to return, the others entered inside to check on him.
Later local people rescued all the five and rushed them to Sadar Hospital, where doctors declared Abdul and Jashim dead,
said medical officer Dr Amanullah.
Kabir and Shahabuddin are recuperating at the hospital, while Nabi has been released after first aid, the doctor said.