Semen from stallions with equal fertility at natural services, but yielding semen with either satisfactory or poor fertilizing capability after freezing and thawing, was processed for scanning electron microscopy before and after freezing and thawing. In all fresh semen samples the following three categories of acrosomal defect were noticed: (1) minor fenestrations of the plasma membrane (PM) and outer acrosomal membrane (OAM), (2) complete vesiculation and loss of PM and OAM and (3) lack of a large circular part of PM and OAM. The frequency of these defects ranged from 15% to 27%. All frozen and thawed samples displayed defects of categories 1-3 at similar frequencies as the fresh ones. However, additional defects categorized as: (4) major fenestrations of PM and OAM and (5) complete vesiculation of PM and OAM without loss of the vesicles were noted upon freezing and thawing. The total frequency of defects categorized as 4 and 5 ranged from 8% to 21%, and they seemed to be more frequent in stallions with poor fertility after freezing and thawing although the difference was not significant. Moreover, a particular defect categorized as (6) loosening of the whole acrosomal cap was found exclusively in stallions yielding semen with poor freezability.