I studied the calling behavior of radio-tagged Mexican Spotted Owls (Strix occidentalis lucida) in northern Arizona. Owls used a variety of calls, with three call types (Four-note Location Call, Contact Call, and Bark Series) accounting for 86% of calling bouts heard. These calls were used by both sexes, but in significantly different proportions. Males (n = 4) called twice as frequently as females (n = 3), and there also appeared to be intrasexual differences in calling rates. Calling activity increased from March through May, then declined from June through November. Calling activity was highest during the 2-hr period following sunset, with smaller peaks 4-8 hr after sunset and just before sunrise. Calling bouts averaged 9.9 min in duration, and were significantly longer when other owls were calling. Owls called more than expected during the last quarter and new moon phases of the lunar cycle, and called most frequently on calm, clear nights when no precipitation was falling. The timing and nature of Spotted Owl calls suggests that calling behavior may be as important in intrapair communication as in territory advertisement. Differences in calling rates among owls suggest that not all owls will be equally detectable using calling surveys.