Variability in acute to chronic ratios (ACRs; median lethal or effect concentration divided by chronic value) has been of continuing interest in aquatic toxicology because of the reliance on ACRs to estimate chronic toxicity for chemicals and species with known acute toxicity data but with limited or no information for chronic toxicity. To investigate the variability and significant differences in ACRs, an extensive data set was compiled of 456 same-species pairs of acute and maximum acceptable toxicant concentrations for metals, narcotics, pesticides, and other organic chemicals. The overall median value for 456 aquatic invertebrate and fish ACRs analyzed in the present study was 8.3, with a 16,000-fold range in values (1.1–18,550) and a 32-fold range in 10th and 90th percentile values (2.5–79.5). Median ACRs for taxa, ambient habitat media, chronic test end point, and chemical mode of action (MOA)/class categories generally were similar but, in some cases, extremely variable (ranges of 1 to gt;10,000). No significant differences (p ⩽ 0.05) were found in median ACRs between taxa, although invertebrate ACRs generally were more variable than fish ACRs. Freshwater organisms had median ACRs significantly greater than those of saltwater species and also were more variable. No significant differences were found in median ACRs among chemical MOA/class data sets; however, ACR variance differed significantly among MOAs. Although few significant differences occurred among median ACRs for different groups, those categories that were highly variable are at an increased risk of underestimated chronic toxicity when mean or median ACRs are used.