Key to species of male Isoplastus of the continental United States and Canada

1. Paired apices of median lobe of aedeagus each divided at tip (Fig. 2); parameres extending beyond apex of median lobe (Fig. 2); widely distributed, including Florida ..... I. fossor Horn

– Paired apices of median lobe inwardly curved, not divided at tips (Fig. 3); parameres reach apex of median lobe (Fig. 3); Florida .................................... I. floridanus Peck and Cook, n. sp.