A clothes vendor and his mother were shot and killed while his father was injured following a dispute with another vendor at a roadside market in Phetchaburi's Khao Yoi district early on Saturday.
Police said the shooting took place at about 5.30am at the Nong Prong market in Moo 5 village in Tambon Nong Prong.
Police said Sitthichoke Losong, 24, was shot dead at the market while his mother, Supassaras Khamthim, 43, died later at the Phra Chom Klao Hospital.
Sitthichoke was shot twice in the chest and once in his right arm.
Police are hunting for a another vendor, identified only as Nui, who allegedly shot the three.
Other vendors said Nui and Sitthichoke, who had stalls facing each other, often quarrelled.
In the latest dispute, Sitthichoke's parents joined him in a verbal dispute with Nui before the parents left and the two vendors continued their heated quarrel.
During the heat of the dispute, Nui pulled out his gun and shot Sitthichoke who tried to run away but collapsed about 20 metres later. Nui then shot the mother, who was standing nearby, and shot Sitthichoke's father, who was sitting in his pickup truck nearby.