SUMMARYThe influence of various factors on the performance of organomercury seed treatments in controlling seed‐borneSeptoria nodorumwas studied in laboratory experiments. Control was least effective on the most heavily infected seed samples. On heavily infected seed ethyl mercuric chloride was more effective than phenyl mercuric acetate, although 5 wk storage of the treated seed before sowing increased the effectiveness of the latter compound. Effectiveness was not generally influenced by differences in soil moisture. Seedling vigour was significantly improved by levels of mercury below those applied commercially. The significance of seed treatment in the epidemiology ofSeptoriadisease is discusse