Historically, Onager or persian wild ass, Equus hemionus onager , were widespread in arid and semi-arid steppes of the central parts of Iran, but today only two natural populations have remained. Onager is endemic to Iran. This suecies is listed as Critically Endangered in the Red List of the International Union for the Conservation of Nature and Natural Resources (IUCN). Management and conservation of this threatened species is generally needed to get an accurate knowledge about its population size and habitat requirements. We study population density and habitat selection by Persian wild ass in Qatruiyeh National park. Habitat selection was studied in autumn, winter, spring and summer, using clearance transect, standing crop pellet group counts and summer direct observation. Line transect distance sampling was used to estimate the abundance of onager in the park. The results showed habitat use was significantly more for Artemisia sieberi than Zygophillum-Artemisia in summer, annual average and standing crop pellet group counts. Minimal models, resulting from multiple regressions, retained a number of habitat variables in each seasons. The most sensible relationships were negative effects of Zygophyllum eurypterum and trough distance. Negligible nutrition of zygophyllum and its relatively high height can consider this negative relationship. Negative relationship to trough distance show importance of water resources for onager presence. Results of summer direct observations showed group size of onagers was significantly more for Artemisia sieberi habitat. Large group size in this habitat can be explained by openness, high visibility and awareness of animal. The estimated population density of onager in Artemisia sieberi habitat was significantly higher than the Zygophillum-Artemisia habitat, but no significant difference was found between the three plains with regard to the onager density. The mean density of onager in the National Park was 1.09 ± 0.27 SE. Keywords: Habitat selection, Equus hemionus onager , Qatruieh National Park, Standing crop, Line transect, Population density
