We present experimental data of acoustic emission (AE) induced by dislocation motion during pure viscoplastic (ductile) deformation of singlecrystals and polycrystals of ice which provide opportunity to revisit collective dislocation dynamics as a critical phenomenon, as recently proposed for brittle fracturing. The data were recorded during compression and torsion creep experiments. AE statistics of power law type were systematically obtained under different experimental conditions. Among the possible candidates for such a system with threshold dynamics exhibiting power law statistics, critical points, disordered first-order transitions, and self-organized criticality should be considered. The revisitation of dislocation dynamics as a critical phenomenon allows rationalization of collective effects as well as of the heterogeneity and complexity of viscoplastic deformation of crystalline materials. Such critical behavior implies that dislocation avalanches and strain localizations are unpredictible, in a deterministic sense, in space, time, and energy domains and that large plastic instabilities account for most of the viscoplastic deformation. ¿ 2000 American Geophysical Union