The primary goal of this study is to quantitatively rank sensitivities of river macroinvertebrates to metals. Using microcosm experiments conducted over a 18 year period and a spatially extensive survey of Colorado streams, we compared responses of 16 aquatic insect families to metals. For the field and microcosm datasets, we determined sensitivity rankings based on an effect concentration of 20% (EC20). Comparisons of field and microcosm-derived EC20s showed that aquatic insects were consistently more sensitive to metals in the field. The most significant findings of this study were the differences in relative sensitivity of aquatic insects estimated from field and microcosm data. Previous field studies have shown that among mayflies, baetids are relatively tolerant to metals, whereas heptageniids are often highly sensitive. Although this trend was observed in the field study, clearly the opposite pattern was observed for these groups based on microcosm data. This presentation will discuss several factors that may contribute to the observed discrepancies between field and microcosm derived responses by aquatic insects.