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We monitored populations of 2 species of desert rodents, the sandy inland mouse (Pseudomys hermannsburgensis) and spinifex hopping-mouse (Notomys alexis), over 18 years in the Simpson Desert, central Australia. Populations fluctuated synchronously from very low numbers, or "busts," during prolonged dry periods to high numbers, or "booms," after heavy rainfall 3 times over the study period. On the basis of observations that food resources expand after rainfall, we predicted that rodents would show increased rates of recapture, fidelity to burrows, and burrow sharing during population increase (boom) phases compared with decline or bust phases, and also reduce their movements and foraging activity in open habitats during population booms. The behavior of both species was similar but not as we had anticipated. Burrow fidelity and numbers of animals per burrow were roughly 2-fold higher during both the population increase and decrease phases as compared with the population-low phase, whereas rates of movement were reduced by about half. As revealed by giving-up density trials, animals foraged less at experimental food patches during population increase and decrease phases than during busts, and also foraged less in open than in covered habitats. Recaptures of N. alexis were similar across all population phases, whereas P. hermannsburgensis was recaptured more often when populations were decreasing than at other times. The results suggest that both species are dispersed and highly mobile during bust periods but sedentary and more social during population increases and collapses. These changes in movements and social organization appear to be unusual in desert rodents, and we propose that future studies seek to identify the roles of food and other factors in driving them. DOT: 10.1644/09MAMM-S-205.1.