The wood anatomy of lightweight-wooded tree species growing in a seasonally flooded igapo forest of southern Venezuela was investigated. Thirteen species in five families were studied. The formation of lightweight wood (wood with low specific gravity, SG) occurred primarily at the base of the stem, which experiences the lengthiest inundation, and SG increased with increasing stem height up to a SG typical of terra firme congeneric taxa. The reduction in SG was accomplished by two anatomically distinct methods among these thirteen species. In Pachira aquatica (Bombacaceae), low SG was achieved by increasing the size and number of vessels and reducing the number of fibers per square millimeter. At the base of one sample, the SG was 0.22, vessels occupied 54% of the area, and fibers occupied 7%. At the top of that sample, the SG was 0.51, 21% of the area was occupied by vessels, and 24% by fibers. The size and shape of the cells were not appreciably altered by stem height. In the remaining taxa, low SG was achieved by an opposed process. At low stem heights, most taxa had a lower proportion of vessel area and a higher proportion of fiber area per square millimeter. In Micropholis sp. nov. (Sapotaceae), the SG at the base of one sample is 0.18, 2% of the area is occupied by vessels, and 85% by fibers. At the top of that sample, the SG is 0.63, 14% of the area is occupied by vessels, and 62% by fibers. The fibers at the base of the stem, however, are drastically altered in size, shape, and wall thickness such that they resemble, qualitatively and quantitatively, vessel elements at the same stem height. As stem height and SG increase, the fibers become increasingly more like fibers typical for terra firme woods.

Key words: fibers, flooded forest, specific gravity, wood anatomy