Hunter kill records maintained by the Virginia Commission of Game and Inland Fisheries have indicated that continuous declines of cottontail rabbit (Sylvilagus floridanus) populations have occurred in some areas of Virginia since the early 1930's. This study was undertaken to determine the factor(s) responsible for these declines. The Fort Pickett military reservation in southeastern Virginia, where declines in rabbit harvest were documented, and the Radford Army Ammunition Plant (R.A.A.P.) in southwestern Virginia, known to support relatively high rabbit populations, were the study areas for this investigation. From aerial photos, it was found that little measurable change in rabbit habitat occurred at Fort Pickett between periods spanning known declines in rabbit harvest. Edaphic, meteorologic and vegetative comparisons between areas indicated higher soil pH and magnesium levels at the R.A.A.P. than at Fort Pickett; higher mean monthly temperature and rainfall at Fort Pickett than at the R.A.A.P.; and little difference in vegetation and cover characteristics between the two areas. Over four times as many trap nights were required to capture rabbits at Fort Pickett as at the R.A.A.P. Comparisons in physiologic and disease parameters of 164 rabbits, 82 from each study area, were made. Over 30 physiologic measurements were recorded from individual rabbits. These measurements included, body and organ weights, fat condition indices, and reproductive and hematologic measures. Twenty-two individual parasitic and infectious diseases were found. Rabbits were infected with from 5 to 14 species of pathogens. Several species of parasites attained significantly greater infections at Fort Pickett than at the R.A.A.P., and six species of parasites attained higher infections at the R.A.A.P. than at Fort Pickett. Several species of parasites attained higher infection levels than any known previous reports. Area, sex and season comparisons of physiologic and disease measurements are presented. In comparison to the R.A.A.P. rabbit population, the Fort Pickett population was found to have high energy stores but low body and organ weights, low total serum proteins, low serum albumin levels and high eosinophil counts. The Fort Pickett population was also found to have extremely low natality. Optimum conditions appear to exist at Fort Pickett for several parasite species and it is postulated that parasitic disease was limiting the productivity and rate of increase of rabbit populations at that locality. Serologic survey of 198 rabbits for tularemia (Francisella tularensis) antibodies was conducted. Five rabbits had agglutinins present. One of these was collected at the R.A.A.P. and four were collected at Fort Pickett. A severe decline in rabbit numbers was seen at Fort Pickett in the summer of 1974. This decline is postulated to have resulted from an epizootic of tularemia. New evidence is presented for a disease virulence theory of tularemia. It is postulated that a low rate of increase in rabbit populations in Virginia and recurrent epizootics of tularemia have been responsible for continuous declines in hunter harvest. / Doctor of Philosophy