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The no‐effect concentration (NEC) is the preferred threshold metric for single‐species toxicity tests applied to derive safe concentration thresholds for contaminants in the environment for use in species sensitivity distributions. However, the NEC is only suitable when concentration–response (C‐R) data exhibit a threshold response. We describe an alternative toxicity estimate, the no‐significant‐effect concentration (NSEC), which is better suited to C‐R data for which the response is a monotonically decreasing function of concentration and no threshold effects are evident. We use a flexible, three‐parameter sigmoidal function to describe the C‐R relationship and detail both Bayesian and frequentist approaches to estimation and inference for the NSEC. While the NSEC is conceptually linked to the traditional no‐observed‐effect concentration (NOEC), it is a substantial improvement over the NOEC because it …