Morphological measurements of Okinawa Rail Gallirallus okinawae were made to evaluate methods of sex determination. Males were significantly larger (P<0.05) than females for 23 of the 25 features measured. Forward discriminant function analysis identified eight of the most useful measurements as maximum wing, exposed culmen, tarsus, total head, gape, bill height, body weight and third toe length. Further analysis selected total head (TH) and bill height (BH) as the most useful variables to determine sex and produced the discriminant function: 0.467×TH 0.827× BH-53.811, which correctly identified the sex of 100% of the individuals in the data set. Bill and iris color proved useful for determining age, as did the tip shape and color of the primary feathers. Characteristics of juvenile primaries were especially useful, since the birds retain their first year plumage until molting at about one year old. Individual variation also exists, however, requiring comprehensive evaluation of all biometrics and features for some individuals.