1.We examined the numerical dynamics in four natural populations of the least killifish [Heterandria formosaAgassiz (1855)]; to ascertain how those dynamics affected the expression of key life‐history traits and, in turn, whether life‐history variation among populations might be responsible for different dynamic properties. Populations were chosen from two types of communities, spring‐fed rivers and lakes, to examine the relative influence of community characteristics on these relationships.2.Populations in lakes had lower densities, more female‐biased sex ratios and a smaller proportion of immature individuals than populations in rivers. Predator faunas differed between habitats and the ratio of predator toH. formosadensity was higher in lakes than in rivers.3.Females in lake populations were 35−40% larger than river females. Female size was positively correlated with the number of broods carried by a female at any given time and the number of offspring in a particular brood. These correlations and the larger body size of females in lake populations indicate that the reproductive output per female was higher in lake than river populations.4.Female size was negatively correlated with population density in two of the four populations. After adjusting for the variation in female size, brood size and brood numbers were negatively correlated with population density in only the highest density population.5.Average offspring size was negatively correlated with brood size in all of the populations, indicating a general trade‐off between offspring size and number. The average offspring size in the highest density population was as much as 45% larger than that of all other populations, an effect independent of any phenotypic plasticity in offspring size with respect to female body size or density.6.The effects of density on female body size are the major avenue for negative feedback of population density on the subsequent dynamics through life‐history expression. Whether such an effect is stabilizing or destabilizing cannot yet be determined. If the variation in offspring size among populations is adaptive, it may be a prominent example of density‐dependent life‐history evolution.

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