Abstract
Laboratory trials were carried out with ticks of a northern New South Wales strain which had proved impossible to control by dipping in 0.2 per cent. As2O3. The concentration of arsenic required to prevent 50 per cent. of engorged female ticks of this strain from laying viable eggs was found to be approximately double that for females of a reference strain. The median lethal concentration of arsenic for larvae of the northern New South Wales strain was also found to be approximately double that for larvae of the reference strain.