The biological mode of action of metalaxyl (CGA 48988) against P. parasitica var. nicotianae was investigated on agar and on tobacco [Nicotiana tabacum cv. Hicks] seedlings in petri dishes. The ED50 for mycelial growth on V8-juice agar was 0.2 .mu.g/ml. On water agar the formation of sporangia from chlamydospores was completely blocked by a concentration of 1 .mu.g/ml. In the petri dish-tobacco seedling system, 0.2 .mu.g/ml CGA 48988 prevented the formation of sporangia both from chlamydospores and on infected tobacco roots. When sporangia or zoospores were used as inoculum for tobacco seedlings, CGA 48988 did not inhibit germination or penetration processes; fungus development within treated roots was completely blocked by a concentration of 1 .mu.g/ml. Protective and therapeutic action against black shank in tobacco seedlings was exhibited at 1.0 .mu.g/ml of CGA 48988.