The mean weight of engorged female nymphs was greater than for male nymphs; after feeding on rabbits the mean weights were 51.55 and 33.18 mg respectively, and on rats 50.96 and 34.28 mg. The peak of detachment was seven days after infestation for nymphs of either sex fed on rabbits and eight days for those fed on rats. The final weight of nymphs of either sex was not related to the feeding period. The premouit time at 30°C and 2-3 mm Hg saturation deficit in darkness was longer in female nymphs (19.7 days) than in males (17.7 days). Heavier engorged larvae produced heavier engorged nymphs which, in turn, gave rise to more females than males. Prediction of the sex of nymphs using engorgement weights was unreliable due to weight overlap. The different sizes of male and female nymphs must create problems in developing a technique for accurately evaluating field populations on hosts of bisexual three-host ticks. The different premouit periods and weights of male and female nymphs must be taken into account in experimental designs involving engorged nymphs.