Study of ultrathin sections and freeze-fracture replicas of erythrocytes containing Aegyptianella bacterifera (Rickettsiales; Anaplasmataceae) revealed that the organisms develop within a membrane bound vacuole in the erythrocyte cytoplasm. The organisms divide by binary fission to produce about 12 rickettsiae in a mature inclusion. The organisms have gram-negative cell envelopes. The distribution of intramembranous particles within the outer and plasma membranes of A. bacterifera is similar to that of other rickettsiae and gram-negative bacteria such as Escherichia coli. The definition of the genus Aegyptianella should be broadened to include rickettsiae measuring up to 5 μm in length prior to division which develop within membrane bound inclusions in erythrocytes of their hosts.