The primary objective of this study was to investigate the effect of age, season and the interaction between age and season on the sperm morphological characteristics of a selected population of Friesland bulls kept at an artificial insemination (AI) centre. The study was done at the Taurus AI station, South Africa with 271 Friesland AI bulls aged 12, 24, 36, 48, 60, 72, 84 and 96 months and older over a period of five years. Semen was collected by means of an artificial vagina in all four seasons of the year. Semen samples were screened for the percentage of normal sperm, percentage and total major defects (e.g. knobbed acrosomes, pyriforms, abnormal lose heads, dag defects, nuclear vacuoles, degenerative heads, mid-piece reflexes), the percentage and total minor defects (e.g. normal lose heads, distal droplets, curled end-piece and lose acrosomes). The results indicated that the percentage of normal sperm and major sperm defects were affected significantly by age of the bulls. Bulls at 36 to 48 months of age exhibited a better sperm morphology than bulls older than 72 months and bulls younger than 36 months of age. Major sperm defects that were affected by age included pyriform, knobbed acrosomes, mid-piece reflexes and dag defects. Season of the year also affected the percentage of normal sperm significantly. Percentage of normal sperm and major sperm defects were recorded as 72.8±1.6%, 79.4±2.2%, 82.5±2.4% and 84.4±2.4% and 20.3±1.3%, 15.3±1.9%, 12.5±2.0% and 11.5±2.0% for summer, autumn, winter and spring, respectively. Major sperm defects such as proximal droplets and pyriform differed significantly with season. By contrast, knobbed acrosomes and mid-piece reflexes did not differ between seasons. Even though there was a wide variation in sperm morphology, better morphology was observed in spring and winter than in summer and autumn. Major sperm defects, particularly the prevalence of knobbed acrosomes and dag defects, were affected significantly by the interaction between age and season. Young bulls recorded a higher prevalence of major sperm defects during winter and old bulls a higher prevalence during the summer. It is concluded that age and season contribute significantly to the proportion of variation in the sperm morphological characteristics of Friesland AI bulls in South Africa. For successful AI programmes, semen collection in Friesland bulls should be done at 36 to 48 months of age and preferably during spring or winter.