Tensile experiments have been performed on aggregates of equiaxed and randomly oriented ice Ih of varying grain size (1.4 to 7.9 mm) at −10°C at two strain rates, 10−3 s−1 and 10−7 s−1 . At both rates, which were held constant using “feed-back” control, the tensile strength decreased with increasing grain size. This result confirms earlier work on grain size effects in which the rate randomly varied (from 0.6 × 10−6 s−1 to 3 × 10−6 s−1 ) from test to test. At the higher rate the ice is brittle over the complete range of grain size and its strength is given by the relationship σf = σo + kd−1/2 where σo = 0.51 MPa and k = 0.03 MPa m1/2 . At the lower rate the ice is ductile over the same range of grain size and its strength is given by the relationship σf = Kd−1/2 where K = 0.050 MPa m1/2 . These effects are explained quantitatively in terms of the nucleation and propagation of cracks.