Level of damage caused by freezing and thawing to four spermatozoal organelles (individual mitochondrion, midpiece, nucleus, and perforatorium) and the relationships of the integrity of these organelles in fresh and frozen-thawed semen with fertility were examined. Semen sample from 10th generation males of a line of chickens selected for increased duration of fertility of frozen-thawed semen and the corresponding randombred control line were used. In both the selected and control lines, the freeze-thaw process caused significant (P less than .05) detrimental damage to the ultrastructure of the mitochondria, midpiece, and perforatorium but not to the nucleus. Types of damage were identical in both lines. Granulated nuclei were observed in both frozen-thawed and freshly ejaculated spermatozoa and were referred to as a nuclear defect. This nuclear defect was associated with reduced fertility, the effect being more severe with frozen-thawed semen. Where the incidence of the nuclear defect was greater than 2% in frozen-thawed semen, fertility was found to be very low or nil regardless of the degree of structural integrity of the mitochondria, midpiece and perforatorium. Highly significant (P less than .01) positive correlation coefficients were observed for percentage fertility 2 to 8 days postinsemination and duration of fertility in days with percentages of normal mitochondria (.80 and .92), midpiece (.79 and .87), nucleus (.86 and .94), and perforatorium (.84 and .97) for fresh semen. With frozen-thawed semen, the positive correlation coefficients were significant (P less than .05) for midpiece (.64 and .69) and nucleus (.63 and .71) and nonsignificant for mitochondria (.52 and .50) and perforatium (.20 and .30).(ABSTRACT TRUNCATED AT 250 WORDS)