During a 9-year period, 1938-1946, approx. 18% of the queens used in 1,540 package colonies were superseded abnormally. Investigations of the problem since 1942 indicate that at least 88% of this loss can be attributed to the infection of the queen by the protozoan Nosema apis. The queen may become infected while in the mating nucleus, from infected attendants in the mailing cage, or from infected workers in the package population. In tests made 2/3 of the queens allowed to feed on sugar sirup containing Nosema spores were superseded within 30 days, while the others lasted 46 days. From 30 to 90% of the packages received, regardless of their origin, showed infection on arrival. The degree of infection increased during the first 3 weeks before there were emerging bees. Nosema has depressing effect on brood rearing as shown by the negative correlation coeffs. of [long dash]0.31 and [long dash]0.42 obtained in 1945 and 1946, respectively. Colonies in the low-infection group that retained their original queens averaged 54 lb. more surplus honey in 1945 than those in the high-infection group. They averaged 111 lb. more honey than the colonies in the high-infection group that superseded or had their queens replaced. Methods of reducing Nosema losses by management practices are discussed.