The authors obtained a preparation of phthalofyne (3-methyl-1-pentyn-3yl acid phthalate or 3-methyl-1-penfyn-3yl sodium phthalate) to be used for intravenous injection, and tested its anthelmintic effect on 23 experimental dogs. The following results were obtained.1. Whipworms were removed at a rate of 99.7 and 73.9 per cent when a dose of 200 and 250 mg /kg was administered respectively. Any dose of not more than 150 mg/kg gave no noticeable effect.2. Hookworms were expelled in some of the dogs given a dose of 250 mg/kg. The average rate of removal of hookworms was 8.8 per cent.3. The intravenous application of phthalofyne was not effective at all against tapeworms.4. As side-effects, ataxia appeared in 20.0 and 83.3 per cent of those medicated with a dose of 200 and 250 mg/kg, respectively, and vomiting in two of five dogs administered with a dose of 200mg/kg. Ataxia disappeared one to three hours after medication.5. Five dogs injected with a dose of 250 mg/kg were examined clinically and hematologically. They manifested no particular symptoms. Some of them showed a decrease in erythrocyte count and an increase in leukocyte count. The eosinophile count approached its regular value.6. In ten dogs given a dose of 200 and 250 mg/kg in the field, the rate at which the excretion of whipworm eggs turned to be negative was 75.0 and 83.3 per cent, respectively.These results indicate that this preparation of phthalofyne to be used for intravenous injection is effective against whipworms. The medication of this drug requires no preceding fasting nor succeeding catharsis.