Recent reviews of work on poliomyelitis1 contain no data pertinent to the incubation period of this disease in man. Many persons following Wickmann's2 lead attempted to estimate the incubation period from the occurrence of multiple cases in given families. Aycock and Eaton,3 in studies of multiple cases in more than 500 families, demonstrated that simultaneous infection is the rule in familial poliomyelitis and that the shorter intervals are probably due to a variation in the incubation period in simultaneously infected persons. Since the method of transmission of the disease has not been proved for man,4 the only feasible method of estimating the incubation period directly is to collect a series of instances in which the initial victim in a neighborhood either visited or was visited by a patient with acute poliomyelitis from some other neighborhood. In an extensive outbreak in Walker County, Ala., in the summer