Abstract
SYNOPSIS Coccidia‐free and mite‐free domestic canaries Serinus canarius given sporulated Isospora oocysts of canary origin and autopsied at intervals thereafter had numerous organisms in cells of the macrophage system structurally similar to organisms called Atoxoplasma. Judging from the sequence of intensity of infection, the mucosa of the upper intestine was parasitized first and the organisms spread from there down the intestine and to the internal organs. The prepatent period of 10 days for the Isospora infection could be started in one bird by oocyst administration and completed by transfer of infected liver cells from this bird to a 2nd bird. Isospara oocysts from sparrows were not infective for a canary.