Horning vegetation, an expression of aggression predominately among adult males, may be universal among horned ungulates. We found that horning by wildebeest (Connochaetes taurinus) males had an important impact on the Serengeti ecosystem, Africa, from the 1960s to the 1980s, as the wildebeest population increased from 0.25 million to 1.5 million. Between 1979 and 2003, we sampled 2,626 trees and bushes to assess horning impacts. In the 1986 survey, 57% (n = 1,416) of trees and bushes had suffered moderate to severe horning injury. Severe damage frequency was highest (68%) in open grassland, where a few trees were exposed to many wildebeests, and lowest (24%) inside savanna woodland where wildebeest rarely go. Horning by 300,000-400,000 adult male wildebeest contributed to converting, savanna woodland into tree savanna and open grassland. Horning by wildebeest, in combination with known impacts such as grazing, manuring, and trampling, may result in ecological impacts to Serengeti ecosystems only exceeded by the elephant (Loxodonta africana) and fire. More research is needed to understand the ecological and management implications of horning.