One hundred-and eight-six fungi were isolated from one hundred-and-eighty cases clinically diagnosed as otomycosis. They comprise 59 species of 26 genera of moulds, and 2 genera of yeasts. This large variety of mould isolates provided ample material for in vitro experiments, to evaluate the anti-fungal activity of six antimycotic substances found in the preparations currently used for the treatment of otomycosis. The antifungal activity of clotrimazole and tolnaftate in vitro was evident. For over 94 per cent of the 59 fungus species tested, the MIC was less than 0.1 micrograms/ml, and for 6 per cent it was between 0.4 and 1 microgram/ml. Concerning the other 4 antimycotic substances (iodochlorhydroxyquin, fluonilid, natamycine and polymyxin B sulphate) the MIC ranged from greater than 100 micrograms/ml to 1 microgram/ml for the majority of tested fungi. The clinical observations were mostly in accordance with these findings.