Field observations of the zone affected by a small nuée ardente which issued from Santiaguito volcanic dome on 15 September 1973, have established clearly that the eruption came from the distal end of a blocky dacite lava flow. This emphasizes that such eruptions need not be related to an underground magma storage. The Santiaguito nuée ardente, like previous similar eruptions, resulted in a small avalanche deposit, strictly confined to a river bed, and a broad « hurricane cloud » (ground surge?) deposit, which came in advance of the avalanche material. Grain size analyses of the Santiaguito nuée ardente avalanche deposit and similar deposits from recent activity at Arenal Volcano, Costa Rica and Fuego Volcano, Guatemala show generally similar results: mean grain sizes in the range of 0.5-1.0 mm and standard deviation of 2 Φ units. The deposits are slightly better sorted than most published descriptions of ignimbrites, and SEM study shows they include more than 50% of non-vesicular material (including crystal and lithic fragments). The presence of so much non-vesicular material in the nuée deposit (in contrast to its paucity in many ignimbrites) probably is caused by the higher crystallinity and lower volatile content of the parent magma; one effect of this material is probably to render the nuée ardente less mobile than its ignimbrite counterpart, and quite possibly no more mobile than a cold rock avalanche. The observation of highly vesicular particles with ruptured surface vesicles in the Santiaguito nuée materials supports hypotheses that employ active vesiculation as part of the mechanism of movement for some nuée ardentes.