Long-distance vocal communication exists in many group-living carnivores. Understanding its behavioral and ecological significance suffers from few quantitative studies in undisturbed, wild populations. In Yellowstone National Park, Wyoming, United States, we examined seasonal changes in occurrence of wolf howls and howling replies based on more than 11,000 unsolicited howls given over a 10-year period. Howling was 5-fold most frequent in the pre-breeding and breeding seasons. Pack howls primarily, but also single howls, were most common during these seasons. Answers during these seasons were predominately interpack howls. These howling peaks correlated with elevations in estradiol, testosterone, and luteinizing hormone reported elsewhere. Following the breeding season, overall howling abruptly decreased through March and April, although howling at den sites was frequent, particularly in April and May. Howling frequency remained low all summer, during which time answers switched abruptly and almost exclusively from interpack to intrapack. Single howls stimulated distant pack members to answer with increasing frequency as the summer progressed. Although not independent, the frequency of both total howls and interpack howling rose throughout the fall. We relate these seasonal changes in total howling and interpack answers largely to breeding and spacing behavior in pre-breeding and breeding seasons, and intrapack answers to pack cohesion in other seasons. Because our results may reflect a high-density, unexploited wolf population, comparative studies under other conditions would be useful.