Brucellosis was studied opportunistically in bison (Bison bison) in the free-ranging Jackson herd of approximately 120 in Teton County, Wyoming (USA) in March 1989. Recent abortion was diagnosed in a 2-yr-old cow and Brucella abortus biovar 1 was isolated from vaginal discharge, uterine contents, uterus, and supramammary lymph nodes. Endometritis was characterized by lymphoplasmacytic infiltrates in the lamina propria and neutrophils in uterine glands and within necrotic debris and exudate in the uterine lumen. A 5-yr-old bull had diffuse lymphoplasmacytic infiltrates in epididymis and accessory sex glands; B. abortus was isolated from seminal vesicle and ampulla. Twenty-seven (77%) of 35 bison tested from 1989 to 1990 were serologically positive or suspect on tests for Brucella antibodies. We report the occurrence of abortion due to brucellosis in free-ranging bison in the Jackson herd, suggest that bison in this herd are capable of transmitting brucellosis to other susceptible hosts, and report the first confirmation of brucellosis in this herd.