SummaryMaterial from 100 impala from a mainly grazing population in the Kenya Rift Valley was investigated to establish age criteria applicable in both the field and the laboratory. Methods relate to tooth eruption, replacement and attrition; horn development; skull and long bone growth and ossification.Determinations are related to the main calving seasons, which occur with two peaks at the time of the long and short rains. All results are approximations which have an accuracy of ±2 months up to physical maturity at an age of c. 2 y. Age determination in animals from 3–8 y appears to yield reliable results with an accuracy of ±6 months at the lower end of the scale, and ± 1 y or even 2 y in older ani