Pentastomida generally considered to reach adulthood in the respiratory systems of reptiles may attain precocious development in some of these definitive hosts. Sexually mature Kiricephalus pattoni and Porocephalus crotali have been recovered from serous cavities and tissues. In tissue dwelling forms there is evidence that contacts may be maintained with the atmosphere through integumentary rents. In natural mammalian and reptilian hosts tissue dwelling pentastomes elicit little, if any, pathological tissue response, while in human tissues the response may be highly inflammatory. Studies of extensive collections of Pentastomida made by Kuntz and co-workers from Africa and Pacific islands, especially Taiwan and the Philippine Islands, are the prime bases for this work. However, we are indebted to Drs. J. C. Pearson and M. J. Mackerras, Brisbane, Australia, for tissue forms from the Australian bandicoot and to Dr. R. L. Muller, London School of Tropical Medicine, for specimens from human tissues taken at Ibadan, Nigeria. We are also indebted to the U. S. Naval Medical Research Center, Bethesda, Maryland, for photographs of certain speci-