The effects of the systemic fungicide tridemorph on fungal and plant protoplasts have been investigated. No evidence of reaction at the plasma-membrane was revealed by microelectrophoresis. The fungicide did not affect oxygen uptake by fungal protoplasts but incorporation of labelled histidine was reduced. Fungi-toxicity tests showed that tridemorph had little effect on spore germination but inhibited mycelial growth. It is suggested that the fungicide may act by interfering with the synthesis of proteins essential for hyphal extension.