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The life-history of I. pacificus when fed on guinea-pigs under laboratory conditions is described. Feeding of the female proceeds for 10–11 days. Between 120 and 191 mg of concentrated food is found in the female's gut on detaching from this host, and this is directly related to the unfed body weight of the tick.The egg-laying pattern, in general, shows an initial peak, after which the number of eggs tails off, the whole process lasting approximately 33–40 days. The number of eggs laid by females varies between 790 and 1300. The preoviposition period is about 11–16 days.Larvae hatch in about 53–55 days, and their feeding time ranges from 4 to 9 days, although in the present experiments larvae emerging from early laid eggs took about 10 days to complete engorgements.Moulting of larvae to nymphs at 90% R.H. took 37–38 days, and nymphs fed for 7–11 days, with most becoming replete on the ninth.Very few females emerged from these nymphs, and those that did emerged in just over 30 days. The reasons for this are not clear. No diapause appeared to be necessary to complete the life-cycle, and with due allowance for hardening of the mouthparts the life-history of one generation was about 7 months. Females died within 3 or 4 months if hosts were not available.From the literature and from hitherto unpublished records some 55 vertebrates have been implicated as hosts to I. pacificus; these hosts include a wide range of mammals, six species of birds and possibly nine species of lizards.The species has been listed from British Columbia and from five western states of U.S.A. The need for more intensive ecological work on this species is emphasized.We are indebted to Glen M. Kohls, J. D. Gregson and D. E. Johnstone for permission to use unpublished records from their respective institutions. One of us (D.R.A) is also indebted to the Wellcome Trustees for financial assistance to visit Canada during the preliminary stages of the work.

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