Iridaea cordata cultivated in the presence of UVB radiation (UVBR) was studied using transmission electron microscopy. Apical segments were cultivated in 0.97Wm(-2) of UVBR for 40 days, 3h a day, and compared to a negative control (UVBR absent). UVBR caused modifications, mainly in the cortical cells, including an increased number of cell wall-producing vesicles, in addition to thicker and denser cellular walls, compared to the control. Additionally, cells were observed with an irregular contour and without defined organelles. The increase of cell-wall thickness could be interpreted as an acclimation to UVBR, which could lead to protection from this radiation.