Two dung beetle species were studied over the course of 14 months in grasslands located on the eastern side of the Pico de Orizaba volcano in the state of Veracruz, Mexico, at an altitude of 2,700 m. Field samples were analyzed monthly to determine population abundances and sexual maturity stages. The most abundant species was Aphodius (Trichaphodius) opisthius Bates, which was active from June to December 1998, and from May to July 1999. In 1998, this species showed two population peaks, the highest in June, the other in November. Between June and August, individuals were found in maturing and mature sexual stages. In contrast, between September and December, the individuals found were recently emerged and immatures. No Aphodius opisthius individuals were found between January and April 1999, the period of the year that is cold and dry. Possibly the immature adults of both sexes entered into diapause. In 1999, the species showed a population peak again in the humid month of June. From May to July, individuals observed were found in maturing and mature stages. Cephalocyclus hogei Bates was less abundantly found, being active only in June of 1998 and in June and July of 1999, and presenting just one abundance peak each year. All females observed were mature. Maturing males were found in July of 1998, mature males in June of 1999, and immature, maturing, and mature males in July of 1999. From August 1998 through May of the following year, no individuals of this species were encountered, they may have entered diapause during these 10 months. Both species are univoltine, and the emergences of the new generations in these species do not overlap.