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The Amblyopsidae show a sequence of adaptation to caves where they have no predators and food is the main limiting factor. The sequence is shown, in order, by the epigean species Chologaster cornuta, the troglophile C. agassizi, and the three troglobites Typhlichthys subterraneus, Amblyopsis spelaea, and A. rosae. Eye and pigment degeneration are a time index for length of isolation in caves. Extent of degeneration is correlated with extent of cave adaptation in amblyopsids and other cave fish, with the modifying influence of food supply, number of predators, and sensory endowment of the ancestral species. In the Amblyopsidae the epigean species is preadapted to caves by having small eyes, being nocturnal, and having sensory receptors which allow feeding and orientation in an aphotic environment. However, it cannot survive in an aphotic environment. Its troglophilic relative can survive in an aphotic environment. As cave adaptation progresses eyes become reduced and degenerate and pigment may be lost. At first, eye and optic lobe variance are high but variance decreases as the effects of allometry, accumulation of mutations under relaxed selection, and ontogenetic degeneration become stabilized. Olfactory receptors, neuromast receptors, tactile receptors, and equilibrium receptors and their brain centers become more hypertrophied as cave adaptation increases. Capacity to integrate sensory information, as reflected by ability to detect prey and avoid obstacles or remember their position, also increases. The head becomes larger; fins become longer, and neuromasts more exposed. Absolute growth rate decreases with increasing cave adaptation. Lower developmental rates are. related to larger and fewer eggs. As number of eggs decreases so does rate of population growth. Lower rate of population growth is also related to increasing age at first reproduction and increasing longevity. These changes and irregular reproduction by few of the potentially breeding fish cause a shift of population structure toward adults in the more cave adapted species. Metabolic rates and intensity of reaction to disturbing stimuli decrease with increasing cave adaptation. This does not involve a sacrifice of activity since swimming frequency and efficiency are higher in the more specialized species. Acknowledgments.-This study comprised part of a dissertation submitted in partial fulfillment of the requirements for the Ph.D. at the University of Michigan. The tenure of two predoctoral NSF fellowships and a grant-in-aid from the Horace Rackham School of Graduate Studies, University of Michigan aided this study greatly. Numerous persons have aided in field work and in loan of specimens. Special thanks are due Dr. Thomas Barr, Mr. James Langhammer, Dr. Fritz Whitesell, and Dr. Loren P. Woods. Special thanks are also due my wife Elizabeth, who patiently reviewed and typed many versions of this manuscript, and Dr. William R. Dawson, who provided guidance and stimulating criticism throughout the course of this study.