The Northwestern pond turtle (Actinemys marmorata) is native to Washington State, USA and has developed a grossly evident form of shell disease affecting a large percentage of the free-ranging population in this state. Emydomyces testavorans is a novel fungus in the order Onygenales that is the presumed causative agent for shell disease in the Northwestern pond turtle. Terbinafine hydrochloride is a lipophilic allylamine broad-spectrum antifungal that penetrates keratin and concentrates in the stratum corneum. This study evaluated the drug concentration in the plasma and keratin of 18 Northwestern pond turtles after nebulization with 18 mg terbinafine solution (2 mg/ml) once a day for 28 days. Blood and keratin samples were collected serially during the course of treatment, and for 14 days following the last dose. Plasma and keratin were analyzed by high-performance liquid chromatography. No significant concentrations of terbinafine were found in the plasma of the turtles. Terbinafine in turtle keratin peaked after 16 days of treatment and maintained therapeutic concentrations for 14 days posttreatment. Turtle shell lesions also showed signs of clinical improvement posttherapy. Nebulization of terbinafine is recommended for the treatment of shell disease secondary to Emydomyces testavorans; however, pulse antifungal therapy is likely needed to prevent disease from reoccurring.