Inhibition of net root elongation and patterns of hematoxylin staining were used to assess relative tolerance to phytotoxic Al in Danthonia linkii Kunth and Microlaena stipoides (Labill.) R.Br. According to net root elongation, M. stipoides is significantly more tolerant of phytotoxic Al than D. linkii. In nutrient solutions with Al concentrations of 370 µM and higher, root elongation is stopped in D. linkii after 24 h while in M. stipoides root elongation is maintained at 60–70% of control rates over 72 h. After removal of Al-stress, root growth in M. stipoides from all Al-treatments recovered to be at or above control growth after 72 h. In D. linkii, root elongation in plants exposed to Al levels that caused a reduction in growth (<370 µM), but not complete cessation, recovered after removal of Al stress. Greater intensities of hematoxylin staining were seen in Al-stressed root tips of D. linkii compared to M. stipoides, suggesting that inhibition of root elongation is associated with increased accumulation of Al in root tips. Roots of M. stipoides seedlings exposed to all Al-treatments showed a short band of intensely stained tissue, correlating with the position of the root apex at the exact point of initial Al- exposure. New root growth after this band did not stain with hematoxylin, indicating activation of a mechanism of Al-exclusion in roots of M. stipoides.