| Sumario: | Experiments carried out at the Instituto de Pesquisas Veterinarias "Desiderio Finamor", at Porto Alegre, Rio Grande do Sul, Brazil, to study the degree of resistance of ticks to toxaphene dips, gave the following conclusions:
1. The larvae of the toxaphene-resistant ticks, and yet without feeding blood, were completely destroyed by toxaphene dips within 48 hours, at a concentration of toxaphene begining from 0.07% and up.
2. The metalarvae stage is equally destroyed by dips with a concentration of toxaphene from 0,25%, within 48 homs.
3. The same happens to the nymph stage.
4. It is in the metanymph stage that begins the toxaphene-resistance of the ticks Boophilus microplus (Canestrini, 1888)-about 25% of the metanymph stage are toxaphene-resistant
5. The teleogyna stage and gonandro stage are toxaphene-resistant in about 30%.
6. The eggs from the toxaphene-resistant teleogyna stage have a viability of about 79%.
7. The resistance character is hereditary.
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