Pregnant mice were infected with a strain of T. congolense which produces a chronic infection, to determine if congenital infection can occur. Some of the mice were killed before delivery and tissues of foetuses injected into clean male mice. Other mothers were allowed to deliver and the tissues of some of the 1-day-old young inoculated into male mice while the remaining members of each litter were suckled by the infected mother until weaning. Some but not all males infected from foetuses and 1-day-old young became infected, while the litter-mates suckled normally by their mothers did not show any infection. It seems that T. congolense, though not commonly found in tissues of the host, can cross the placental barrier and infect the foetus, but that young suckled by infected mothers may be protected from infection.