It has been found that temperatures ranging from 22 to 38 degrees C are optimal for the development of Argas persicus. Under such conditions the average duration of the whole cycle ranges within the limits of 41 to 133 days. Under conditions favourable for the development of the parasite certain irregularities in the nymph stage of the individual generations have been observed. In some of these up to 97 per cent of the second-stage nymphs transform into imago. This is said to be the main reason for the faster propagation and distribution of the ticks. Adult females that lay eggs for the first time are of considerably higher potential fertility than those that have already passed through several gonotrophic cycles. Under optimal conditions the biologic cycle of the parasite does not discontinue during the whole year. It has been demonstrated that in each feeding the adult ticks engorge about 30 mg of blood and can thus endure fasting for more than 8 years.