Northern rodent populations often exhibit temporal dynamics due to seasonal changes in demographic processes such as survival, reproduction, and movement. Seasonal patterns in their demography partially result from seasonal changes in climate and resource availability. We studied the population ecology of Mongolian gerbils (Meriones unguiculatus), a social rodent living in groups year-round in desert grasslands of China, Mongolia, and Russia, using capture–recapture methods to investigate seasonal patterns in demography. Gerbils were livetrapped biweekly from late April to late October 2006 in Inner Mongolian grasslands, China. We used robust-design models and Cormack–Jolly–Seber models to estimate population size and apparent survival probability. Additionally, we used multistate models to test for a trade-off between reproduction and survival. Like other northern rodents, Mongolian gerbils showed a single annual peak in abundance, but gerbil numbers peaked unusually early in June. Gerbil populations were female-biased and also biased toward older individuals. The breeding season was restricted to the wet season from April to the end of August, and survival declined from April to October. We found a trade-off between survival and reproduction in males but not in females. Kinship and cooperation among females may enhance survival to offset the cost of reproduction.