The effects of trivalent and hexavalent chromium on root growth, cell division and chromosome morphology of ANium cepa were studied. The concentrations used of chromium nitrate and potassium dichromate were 2 x lo-', 2 x 2 x loW3, 2 x loW4, 2 x loW5, 2 x 10W6, and 2 x 10W7M. The results showed that both trivalent and hexavalent chromium inhibit root growth and cause mitotic irregularities comprising c-mitosis, anaphase bridges, chromosome stickiness, and chromosome fragmentation and lagging. When compared with chromium nitrate on the equimolar basis, potassium dichromate showed a stronger inhibitory and toxic effect on root growth and cell division, respectively. The possible mechanism of the Cr poisoning of root meristem cells of Allium cepa is also briefly discussed. Donghua Liu and Wusheng Materials and methodsHealthy and equal-sized bulbs were chosen from a population of the common onion Allium cepa L. (2n = 16). The onions had neither started shooting of green leaves nor started any growth of roots. Before starting the experiment, the outer dry scales of the bulbs were removed and the ring of the root primordia was left intact. 12 onions were the starting material in each series and the best 10 onions were selected on day 1 or day 2 for the continued test ( FISKESJ~ 1988).The concentrations used of chromium nitrate (Cr(NO,), . 9H20) and potassium dichromate (K,Cr,O,) were 2 x lo-', 2 x ]OW2, 2 x lo-,, 2 x 2 x 2 x lop6, and 2 x lO-'M. The solutions were prepared in tap water. Control roots were treated with tap water. The onions were placed directly in the test liquids, which were changed regularly every day. The bulbs were allowed to germinate producing roots in beakers at room temperature, 21"C, for 24, 48, 72, and 96 h. Macroscopic observations were made at the end of each time interval. The roots were cut and fixed in Carnoy's fixative (3 parts 95 % ethanokl part acetic acid, glacial) for 48 h, followed by squashing