The effect of human disturbance on wildlife is of increasing interest because of the growing use of wildlands by humans for recreation. Few studies have documented the effect of human disturbance on behavior and physiology simultaneously, with no studies existing for any turtle species. Turtles are one of the most endangered taxonomic groups and many are of conservation concern, including the yellow-blotched sawback (Graptemys flavimaculata), a freshwater turtle of the Pascagoula River system, Mississippi, USA. We studied G. flavimaculata individual- and population-level basking behavior, while also documenting the effects of human disturbance on basking behavior at recreationally disturbed and control sites. We also assessed the physiological response of turtles to human disturbance by measuring heterophil/lymphocyte levels (H:L; higher levels indicate increased stress) and shell condition of captured turtles at the 2 sites. At the individual level, disturbed turtles at the recreationally disturbed site basked for significantly shorter durations than undisturbed turtles at the same site and undisturbed turtles at the control site; disturbed turtles at the control site basked longer than all groups possibly because the few instances of disturbances all occurred during a time of year when basking durations were longest. At the population level, we detected significantly lesser basking percentages at the recreationally disturbed site relative to the control site, possibly because of natural differences among the sites (i.e., a more stable thermal environment) or because of the higher level of human disturbance. At the recreationally disturbed site, more disturbances occurred on weekend and weekend bordering days relative to weekdays, and larger and slower boats disturbed significantly greater percentages of basking turtles compared to smaller and faster watercraft. Further, turtles from the disturbed site had significantly higher H:L levels relative to the undisturbed site, and an index of shell condition was significantly poorer at the disturbed site. Boating records indicate that the impact of recreational boating at the disturbed site likely has grown over the last 22 years because of an increase in the number and size of boats using the river; this trend will likely continue unless restrictions are enacted by managers and/or state entities to limit the number and size of boats that access the river. © 2013 The Wildlife Society.