Morphological measurements on six well-defined volcanic flows near the summit of the Martian volcano Ascraeus Mons were used to calculate the yield strength and viscosity of the lavas. The yield strength estimates range from 3.3¿103 to 8.3¿104 Pa (N m-2), with an average value of 2.1¿104. These values are similar to yield strengths calculated for other flows on Martian and terrestrial sheild volcanoes, but they are an order of magnitude less than the value for terrestrial andesite flows and two orders of magnitude greater than values for lunar flood basalts. Ascraeus Mons flows indicate lower limits on the effusion rate of 18--60 m3 s-1, with an average value of 35. Effusion rate and yield strength are used to calculate upperlimits as the lave viscosity of 6.4¿105 to 2.1¿108 Pa-s, with an average value of 2.3¿107 (equivalent to 2.3¿108 noise). Ridges on one flow indicate a viscosity lower limit of 1.1¿108 Pa-s for a ridge growth time of one hour. The estimated rheologic properties of the Ascraeus Mons flows are most consistent with basaltic or basaltic andesicte lavas, but some individual flows may consist of more highly evolved lavas.