Morphological and rheological measurements were made on some of the deposits of the pyroclastic flows of July 22 and August 7. Evidence indicates that the flows moved as non-Newtonian fluids most readily modeled as Bingham fluids. Values are given for yield strengths of the flows: at the time of emplacement, after a few days, and after a few weeks. Average values are about 600, 1500 and 10 000 N/m², respectively. Extensive reverse grading of coarse, low-density pumice clasts in the levee deposits was the result of interaction between the flow and the atmosphere, which increased the degree of fluidization of the levees at the time of their emplacement.-from Authors