The investigation reported here involved an analysis of some of the biological factors affecting tonic immobility in young Japanese quail. Both duration and incidence of tonic immobility were measured on each bird at six days and again at 12 days of age. A significant line effect was found at both ages, and rankings of these lines for duration and incidence followed a similar pattern. Number of inductions and duration of immobility were inversely related, while the percentage incidence was positively associated with duration of immobility. A large maternal effect found at six days of age disappeared by 12 days of age. Additive genic effects were low to moderate which is indicative that tonic immobility is associated with fitness. / Master of Science