Screening identified two bisindolylmaleimides as 100 μM inhibitors of the GTPase activity of dynamin I. Focused library approaches allowed development of indole-based dynamin inhibitors called dynoles. 100-Fold in vitro enhancement of potency was noted with the best inhibitor, 2-cyano-3-(1-(2-(dimethylamino)ethyl)-1H-indol-3-yl)-N-octylacrylamide (dynole 34−2), a 1.3 ± 0.3 μM dynamin I inhibitor. Dynole 34−2 potently inhibited receptor mediated endocytosis (RME) internalization of Texas red-transferrin. The rank order of potency for a variety of dynole analogues on RME in U2OS cells matched their rank order for dynamin inhibition, suggesting that the mechanism of inhibition is via dynamin. Dynoles are the most active dynamin I inhibitors reported for in vitro or RME evaluations. Dynole 34−2 is 15-fold more active than dynasore against dynamin I and 6-fold more active against dynamin mediated RME (IC₅₀ ∼15 μM; RME IC₅₀ ∼80 μM). The dynoles represent a new series of tools to better probe endocytosis and dynamin-mediated trafficking events in a variety of cells.