In fire-prone environments, studying the variability of regeneration traits is important to show the potential of plant species to survive under consecutive post-fire and fire-free conditions. We studied the relationship between seed size and germination patterns in Cistus salviifolius (Cistaceae), which is a fire-follower species distributed throughout the Mediterranean Basin. Since fire plays an important role in the life cycle of the studied species, we made a germination experiment that included a heat-shock treatment (to simulate fire) and a control without a heat-shock (to simulate fire-free conditions) using seeds collected from individual plants to assess within-population germination responses to fire. Seed germination of most of the individuals was stimulated by heat-shock, but the germination characteristics showed substantial variation among individuals within the population. Mean seed mass (i.e. seed size) was positively correlated with germination percentage in the heat treatment, but no correlation was found between seed mass and germination percentage in the control. On the other hand, mean seed mass was negatively correlated with seed mortality both in the heat-shock treatment and in the control. The results show that even if the response of the majority of the individuals in the population follows the population-level germination response of the species, a significant variability in germination actually exists within this population (among individuals). Individuals with larger seeds may be favored in post-fire conditions, while seed size is unimportant in fire-free conditions. Together with the positive selective pressure of seed predation over small-seeded individuals, fire shapes the within-population variability of the regeneration traits such as seed size and post-fire germination in C. salviifolius. This variability might have evolved as a bet-hedging strategy against unpredictable conditions in the Mediterranean environment, especially the occurrence of fire and fire-free periods.