A coal petrographic study sediments, including coals, oil shale, and oil source rocks, in the fossil fuel deposits of northern Thailand revealed changes in alginite asscoiations. In the Lower part of these Tertiary deposits, especially in the Fang oilfield, alginite A (a Botryococcus sp.) was the only type of alga found. Later, the association of Botryococcus braunii, Pila algae, thick-walls alginite B, and temperate palynomorphs were recognized in many coalfields, as well as in the middle part of the deposits in the Fang Basin. Their ages were Late Oilgocene(?) to Early Miocene. In the upper part of the fossil fuel deposits, alginite B is dominant in many basins, together with Botryococcus -related taxa such as Pila algae, Reinschia and fresh-water-dwelling ferns. In the Mae Sod Basin Reinschia was found to be dominant in the northern part, whereas lamaginite dominated in the south, showing different environmental conditions in different parts of the basin during deposition. These different associations indicate changes in depositional environments in northern Thailand, resulting from climatic and/or sea level changes during Tertiary time. ã 1999 Elsevier Science Ltd. All rights reserved.