Oral doses of "Haloxon' given to sheep and cattle have a very high efficiency against adult worms of the genera Haemonchus, Ostertagia, Trichostrongylus. Cooperia and Strongyloides and a high level of efficiency against Nematodirus and Bunostomum. Immature stages of some of these parasites are destroyed. The recommended dose for sheep and cattle is in the range 35-50 mgm./kgm.; but much lower doses are effective against Haemonchus spp. and Cooperia spp. 'Haloxon' has a marked anthelmintic effect against Capillaria obsignata in poultry, oxyurids, Nemato-spiroides dubius and Strongyloides ratti in rodents and against ancylostomes and ascarids in carnivores. 'Haloxon' is much less toxic than the related compound 'Coroxon', (I, E = C2H5-), the substitution of a chlorine atom in the ethyl groups of 'Coroxon' having a very profound effect on anticholinesterase activity. 'Coroxon' in glycerol formal shows an acute oral LD50 to rats of 12 mgm./kgm., but for 'Haloxon' in a similar formulation the figure is increased to 900 mgm./kgm. Lambs and yearling sheep have tolerated four to six times the anthelmintic dose. Others have received twenty consecutive weekly doses of 100 mgm./kgm. of 'Haloxon' and have shown no ill-effects. A single oral dose of 250 mgm./ kgm. 'Haloxon' to sheep results in a slight depression of red cell cholinesterase activity (which returns to normal within one week of dosing), whereas an oral dose of 10 mgm./kgm. 'Coroxon' results in a 95 per cent depression in red cell cholinesterase activity.