Population dynamics of the lizard Tropidurus torquatus was studied between May 2008 and June 2009 in a rocky formation in Alegrete municipality, Rio Grande do Sul state, southern Brazil. The study was carried out using the capture, marking and recapture method, the area being randomly searched from 08:00 am to 6:00 pm. Tropidurus torquatus presented variation in population structure throughout the study period, with maximum biomass observed in October 2008 and maximum density occurring in November 2008 (reproductive season), both having a second peak in March 2009 (recruitment period). There was a significant difference between the number of juveniles and adults recorded, since adults were present throughout the study period while juveniles were present in the months after recruitment. The difference found between males and females may be related to a social territorial behavior of the males. The population of Tropidurus torquatus presented a cyclic and seasonal variation in population structure, possibly associated with the reproductive cycle of the species, with differences in the distribution of age classes and sexual proportions throughout the year.