A tour bus with 51 employees of the Royal Irrigation Department's Chiang Mai office overturned in Mae Tha district of Lamphun early Monday, injuring 36 people, police said.
Pol Lt Sakchai Pornchermkul, the duty officer of Mae Tha district police station, said the accident occurred about 10 minutes after midnight between kilometres 508-509 on Lampang - Chiang Mai road in tambon Thasopsao.
The bus was going uphill when it slid off the side of the road and overturned.
Thirty six people - 21 men and 15 women - were injured. Only Sompop Suthirak, 57, the driver, was in a serious condition as he sufferred a broken collar bone and was admitted to Mae Tha Hospital.
The passengers were officials of the irrigation office of Chiang Mai returning from Phetchaburi after a study tour.