The species Phaleria testacea Say, 1824 (Coleoptera, Tenebrionidae) varies a lot in size, and its populations along the coast of Brazil, where it lives buried in sandy beaches, have temporal and geographical variation in density. In order to find out which factors were contributing to these variations, we sampled populations from three different areas in Rio de Janeiro, during summer and winter. Simultaneously, we made an analysis of the different substracts where the individuals were found, including their total fresh weight. The results show a significant difference in size of individuals between and within populations, when summer and winter were compared. There is also a positive association between the population density of P. testacea and the amount of "garbage" found at the study areas.