AbstractTricyclazole (EL 291), 4,5‐dihydro‐4‐methyltetrazolo [1,5‐a] quinazolin‐5‐one (PP 389), and pyroquilon (CGA 49104) were studied to determine the effects on growth and melanin biosynthesis byPyricularia oryzae in vitro.The three fungicides were essentially devoid of toxicity toP. oryzaeat concentrations up to 50 ug ml−1but each selectively inhibited melanin biosynthesis at much lower concentrations. Inhibition of melanin biosynthesis resulted in the accumulation of 2‐hydroxyjuglone and flaviolin. The study indicated that the three compounds act by a similar mechani