Both ovaries from 88 bovine fetuses in the fifth month or later of gestation were studied histologically to determine the prevalence, origin, and time of appearance of atretic corpora lutea (ACL). Ovaries from 36 (41%) fetuses had ACL; fetuses < 6 months of gestation did not have ACL. Six fetuses had more than 25 ACL, but there was no apparent relationship between fetal age and number of ACL. Formation of ACL involved disintegration of the stratum granulosum of secondary follicles, concomitant with proliferation and invasion by vascularized elements of the theca. Fully developed ACL consisted of a large primary oocyte surrounded by a prominent zona pellucida and encased in a well-vascularized, largely thecal, fibrocellular wall. They measured approximately 0.5 to 1.0 mm in diameter. Empty, collapsed zona pellucidas were seen in many of the degenerating ACL.