Abstract: This paper deals with the ecotoxicology of floatation agent Petassium n-butyl xanthat (PNX) to Chironomus plumosus and Limnodrilus hoffmeisteri. The results show that PNXin various concentrations show a consistant toxicity to the selected chironomid larvae and tubifieid. The 24h-LC₅₀ of C.Plumosus is 256.2 ppm and that of L.hoffmeisteri is 25.8ppm. Ecological factors as temperature, pHand salinity may markedly modify the acute toxic effect of PNXto tubifieids. An objective symptom of the two species is successfully proposed in testing.

Key words: Chironomus plumosus, Limnodrilus hoffmeisteri, Petassium n-butyl xanthat, Ecotoxicology