Black and white colobus monkeys (Colobus guereza, “guerezas”) show no external signs of estrus, and little is known about their mating behavior or reproductive endocrinology. To learn more about the life history characteristics and mating system of guerezas, we documented the mating behavior and ovarian hormone profiles of 10 cycling, lactating, and pregnant females in a wild population. We studied six groups of guerezas in Kibale National Park, Uganda, and collected ad libitum data on their mating behavior. We collected urine samples every 1.9±0.2 days from potentially fertile females and quantified conjugated urinary estrogen (E1S) and progesterone (PdG) metabolites using enzyme immunoassay (EIA). Females solicited 50.5% of copulations, and most copulations occurred during the interval starting 5 days before presumptive ovulation and ending 2–3 days later, with the highest copulatory levels occurring close to ovulation. We show that the median ovarian cycle length is 24 days, the median gestation length is ∼158 days, the length of lactational amenorrhea is at least 7–8 months, and the median interbirth interval for females with surviving infants is ∼22 months. We also document overlap between females' receptive periods in one-male, two- to three-female groups, which may mean that male transfer into such groups is potentially profitable. Am. J. Primatol. 68:383–396, 2006.© 2006 Wiley-Liss, Inc.