The influence of metal-complexing agents on the mycelial growth rate, conidial germination and morphology of Aspergillus niger in shake-flask cultures was studied. The agents tested were: ethylenediamirietetraacetic acid (EDTA), diaminocyclo-hexane-N, N-tetra-acetic acid (CDTA), diethylenetriaminepenta-acetic acid (DTPA), and nitrilotriacetic acid (NTA), which form soluble complexes, and ferrocyanide, which forms insoluble complexes. The agents were added singly to culture media in concentrations up to 9.4 m[image]. The sum ot the concentrations of the complex-forming metals (Fe, Cu, Zn and Mg) added to the medium was 1.04 m[image]. At pH 6.5 mycelial growth rate was almost unaffected by any of the agents when they were added after the germination of conidia. At pH 3.5 ferrocyanide at 84 I[mu][image] increased the mycelial doubling time by 30%, but the other agents had little effect on the mycelial growth rate at pH 3.5. EDTA, CDTA and DTPA at 1.04 m[image] inhibited germination of conidia at pH 6.5; lowering the pH partially relieved this inhibition. Ferrocyanide at 9.4 m[image] was without effect on germination at pH 6.5, but at pH 3.5 it strongly inhibited germination at 84[mu][image]. The morphology of the organism was markedly affected by EDTA, CDTA, DTPA and ferrocyanide but not by NTA. In cultures without complexing agents or with NTA the organism grew in the form of large pellets with a filamentous periphery; such pellets formed a gelatinous conglomerate in the culture. With either EDTA, CDTA or DTPA at 1.04 m[image] or ferrocyanide at 84 [mu][image], the organism grew in the form of small pellets with a smooth exterior; such pellets remained discrete and dispersed in the medium. The physiological effects are attributed to changes in the metal ion concentrations in the medium brought about by the complexing agents.