To clarify the relationship of zinc and auxin on root formation, the effects of zinc and auxin on super-growing root (SR) from the legume bird's-foot trefoil (Lotus corniculatus L.) were investigated for 4 weeks. 1) When SR was treated with zinc, the fresh weight of SR increased with the increase of zinc application from 0 up to 0.09 mmol L^<-1>. It showed a peak at 0.09 mmol L^<-1> and decreased with the increase of zinc application from 0.15 to 0.3 mmol L^<-1>. The zinc concentrations in SR at harvesting time were 32.9, 217, 798, 1060 and 1770 mg kg^<-1> at 0, 0.03, 0.09, 0.15 and 0.3 mmol L^<-1> treatments, respectively. 2) As for SR treated with auxin (2,4-D), 0.01 mmol L^<-1> of 2,4-D showed a slight increase in lateral root formation. However, there was a decrease when 2,4-D application exceeded 0.01 mmol L^<-1> for the length of the primary root and the number of lateral roots and took a turn for callus formation at 0.15 to 0.1 mmol L^<-1>. 3) The increased growth of SR with zinc was caused by the increased the number of lateral roots. It was found that the effect of zinc on SR is similar to that, of auxin. 4) Other metals, such as Ni, Co and Ag, and the counter ion had no effect on the growth of SR. These results indicate that the physiological function of the zinc is not involved in auxin synthesis but in root formation, which was one of auxin physiological functions, in the presence of auxin.