Summary: This report presents the results of five generations of selection for resistance to a standard infection with the fowl typhoid bacterium in the chicken. A marked increase in the resistance of the selected population has resulted, the observed mortalities in the selected stocks, from the first to fifth generations, being 39.8, 29.3, 15.4, 15.0 and 9.4 per cent. In the unselected (control) populations tested concurrently the respective mortalities were 89.6, 93.2, 86.2, 86.4 and 85.0 per cent. A combined total of 3355 chicks, 1999 in the selected and 1356 in the control stock, was used in these studies. Reciprocal crosses of selected (resistant) with unselected birds demonstrated that the male, as well as the female, transmits resistance to the offspring; and that a passive transfer of immunity was not a great, if existent, factor in the enhanced resistance of the selected progeny.