The instantaneous rate of population increase (ri) integrates several life cycle variables into one accessible statistic and has been proposed as a more practical alternative than assembling full life tables in the study of population-level responses to toxicant exposure. In this study the sensitivity of instantaneous rate of population increase is compared to critical-effect estimates for populations exposed to four toxicants with different modes of action. Populations of the Collembolan Folsomia candida were exposed to cadmium, copper, pyrene, and chlorpyrifos in artificial soil following the standardized ISO (International Organization for Standardization, Geneva, 1999) protocol. We calculated ri values and LC(50), EC(50juvenile), and NOEC values for each chemical. Comparison of the relative toxicity of the four chemicals indicated that chlorpyrifos had the lowest values and was thus the most toxic, followed by pyrene, cadmium, and copper. Significant changes in ri were seen to follow closely changes in the sublethal parameter measured (juvenile production) and showed populations in decline at concentrations as low as 40% of the LC(50). The study showed ri to be a good measure of population response, and we conclude that the statistic gives a better understanding of effects on a population than through the sole use of traditional critical-effect estimates.