Observations were made on ova recovered from mated, from inseminated and from unmated sows killed at varying intervals up to 144 hr after the onset of heat. The morphological findings on penetrated, pronucleate and cleavated ova are described. The timing of the events that precede and follow fertilization is examined. The cumulus cell mass (egg-plug) was found around ova only from sows killed less than 8 hr after mating. Spermatozoa were not found in the cumulus until 5 1/2 hr after mating. Penetration and pronucleus formation occurred not earlier than 6 hr after mating. The first cleavage in normal mated pigs occurred 21 hr after mating. Some cleaved ova were recovered earlier than this from sows after "delayed" mating. The distribution of ova at different stages of cleavage is recorded for sows killed at varying intervals after mating or insemination. Three of a total of 1677 ova resembled primarily oocytes; all other detectable abnormalities could have arisen in originally normal ova. The number of spermatozoa on the zona pellucida tended to increase with advancing stages of development, but counts of spermatozoa on ova at the same stage of cleavage recovered at different intervals after mating furnished no evidence that cleavage rate is increased by increasing sperm numbers. In unfixed ova from unmated sows, spontaneous fragmentation was observed with increasing frequency after 72 hr from the onset of heat; degenerative changes were identified in fixed and stained ova 48 hr after the onset of heat. Cleaved (fragmented) ova from unmated pigs never showed a normal nuclear pattern. The findings are discussed.