This study addressed the environmental factors that affect Trentepohlia spp. in the Chiang Dao Wildlife Sanctuary at altitudes of 399 to 1,503 meters above sea level (m a.s.l.) during the rainy, winter, and summer seasons. Species were identified using characteristic morphological identification. The influence of environmental factors on the algae was analyzed using a statistical program, and seasonal changes in the quantity of photosynthetic pigments in the dominant species were evaluated. The average relative humidity was 69.34 [+ or -] 12.90%, the average temperature was 26.23 [+ or -] 3.79 [degrees]C, and the average light intensity was 139.78 [+ or -] 42.21 [micro]mol photon [m.sup.-2] [s.sup.-1]. Thirteen species were found: Trentepohlia chapmanii, Trentepohlia sp. 1, Trentepohlia sp. 2, Trentepohlia sundarbanensis, Trentepohlia sp. 3, Trentepohlia rigidula, Trentepohlia sp. 4, Trentepohlia effusa, Trentepohlia monilia, Trentepohlia abietina, Trentepohlia sp. 5, Trentepohlia aurea, and Trentepohlia umbrina. The largest number of species (seven to nine) were found at lower altitudes, from 473 to 517 m a.s.l. Species diversity was greatest in the winter season (13 species). Species found at low attitude were grouped together (Group 1) and had the greatest diversity, and the remaining species were divided into Groups 2, 3, and 4. Environmental factors had both positive and negative influences on the species, especially T. chapmanii, which was found below 1,003 m a.s.l, and T. monilia, which was found in areas with a high relative humidity of 74.50% to 83.93%. The ratio of the total carotenoids to chlorophyll of T. rigidula, the dominant species, was relatively high at 4.96:1, and the [beta]-carotene content (46.89 %w/w) was highest during winter. Keywords [beta]-carotene; Chiang Dao Wildlife Sanctuary; Chiang Mai Province; diversity; genus Trentepohlia; photosynthetic pigment; subaerial green algae; Trentepohlia rigidula