The immature and adult stages of Potamyia flavata Banks 1934 were sampled in seven sampling sites in streams of western Thailand. The samples were collected in February, May, and December 2015 using hand picking. A total of 2,133 individuals of larvae were collected. The larva and pupa of Po. flavata are described and illustrated. Larvae have five instars. The head capsule width of the first to the fifth instar larvae were 0.20–0.29, 0.30–0.39, 0.40–0.59, 0.60–0.79, and 0.80–1.15 mm, respectively. Gut content analysis revealed that larvae are omnivorous filterers. The guts of the larvae contained mainly diatoms and green algae followed by filamentous algae, detritus, and arthropod fragments.