Models of many magmatic processes require accurate data on the rheological properties of lava at sub — liquidus temperatures. Laboratory measurements of the rheological properties of basalts erupted on Mount Etna in 1983 were made at various crystal concentrations in a specially designed furnace using a Haake Rotovisco viscometer attached to a spindle designed to eliminate slippage at the melt-spindle interface. Measurements were made at strain rates between 0.3 and 5 s−1 over the range of eruption temperatures on Mount Etna (1084–1125 °C). At temperatures above 1120 °C, the 1983 lava behaves as a Newtonian fluid. At lower temperatures, the lava is a thixotropic, pseudoplastic fluid with a maximum yield strength of 78 Pa at a temperature of 1087 °C. In view of its low yield strength over this temperature range, the Theological behaviour of the lava approximates to that of a power law fluid. Apparent viscosities at unit shear strain rates increase from 150 Pa s at 1125 °C to 3000 Pa s at 1084 °C. Unit strain rate apparent viscosities measured in the field using a rotating vane viscometer range from 1385 Pa s to 1630 Pa s. These values are in close agreement with those measured at the same temperature in the laboratory and with those calculated theoretically from the physico-chemical properties of the lava.