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Abstract
The prevalence and level of thiabendazole resistance of sheep nematodes in the North Island of New Zealand was investigated by means of an in vitro egg-hatch technique; samples from sheep on 52 properties in six country areas were examined. Resistance of Haemonchus contortus and Trichostrongylus spp. was identified on the basis of egg hatch in 0.1 ppm thiabendazole. Evidence of resistance was obtained on 11 (21.2%) of the properties. LD 50's and resistance ratios were calculated for the resistant nematode populations found.