Transmission electron microscopy was used to investigate ultrastructural changes in root cap cells of two aluminium-tolerant native grass species, Danthonia linkii Kunth and Microlaena stipoides (Labill.) R.Br., following exposure to Al. Quantitative differences in root cap cells and organelles in response to 0–10 ppm aluminium were determined using image analysis of electron micrographs. Changes to the size of root cap cells due to exposure to Al were similar in the two species with a low Al concentration (1–2 ppm) resulting in larger cells, while higher Al levels (5–10 ppm) reduced cell size. In peripheral cap cells of the more Al-tolerant M. stipoides, the size of secretory vesicles was not affected by exposure to Al, while in peripheral cap cells of the less Al-tolerant D. linkii, exposure to Al resulted in significantly smaller secretory vesicles being produced. Central root cap cells from control plants of M. stipoides contained 90% more dictyosomes and had 50% larger amyloplasts than D. linkii. Measurement of mucilage droplets showed that roots of M. stipoides produced much more mucilage than D. linkii. Exposure of roots of M. stipoides to Al in the range 2–10 ppm had no effect on the size of mucilage droplets produced. The possible role of mucilage production in aluminium tolerance is discussed.