The Freshwater Invertebrate Invasiveness Scoring Kit (FI-ISK) is proposed as a screening tool for identifying potentially invasive freshwater invertebrates. FI-ISK was adapted from the Fish Invasiveness Scoring Kit (FISK) of Copp, Garthwaite, and Gozlan, which is an adapted form of the Weed Risk Assessment (WRA) of Pheloung, Williams, and Halloy. Initial assessments using FI-ISK, which include confidence (certainty/uncertainty) rankings by the assessor to each response, were calibrated to determine the most appropriate score thresholds for classifying nonnative species into low-, medium-, and high-risk categories, using both the original medium-to-high risk threshold scores for the WRA (i.e., > or = 6) and for FISK (i.e., > or = 19). Patterns of the assessor's confidence, when making the responses during the FI-ISK assessments, were also examined. Using receiver operating characteristic (ROC) curves, FI-ISK was shown to distinguish accurately (and with statistical confidence) between potentially invasive and noninvasive species of nonnative crayfish (Decapoda: Astacidae, Cambaridae, Parastacidae), with the statistically appropriate threshold score for high-risk species scores being > or = 16. FI-ISK represents a useful and viable tool to aid decision- and policymakers in assessing and classifying freshwater invertebrates according to their potential invasiveness.