A comparative study of the species richness, abundance and phenology of the Scarabaeoidea beetle fauna of the mountains of Santiago Xiacui, Oaxaca, Mexico is presented. Monthly data were obtained during June 2003 to June 2004 in pineoak forests and cloud florests, located at 1,9462,363 m of altitude. Records of 74 species, 36 genera, 23 tribes and 11 subfamilies of Melolonthidae, Scarabaeidae and Passalidae, represented by 4,317 specimens, are included. Onthophagus chevrolati retusus Harold, Deltochilum mexicanum Burmeister, Phyllophaga vetula (Horn), P. ravida (Blanchard), Paranomala inconstans (Burmeister), Xyloryctes telephus and Vindex agnoscendus Percheron, were the most abundant species in the samples. Adults of many species are active during May to August. High species richness was found in the forests, where the species of Phyllophaga, Paranomala, Xyloryctes, Deltochilum, Onthophagus and Vindex, are important consumers of roots, leaves, leaflitter, rotten wood, dung and carrion. Five species of Phyllophaga, Paranomala, Peltotrichius, and Copris are recorded by the first time for the state of Oaxaca.
