Considerable variability can occur in the manganese (Mn) content and concentration of seeds. This variability can influence plant growth and development, crop yield and seed quality. In order to understand which factors affect seed Mn variability, the effects of site of growth, season and genotype on the Mn content of barley seed were examined. Plant-to-plant and within-plant variability of white lupin seed were also examined. Manganese concentrations of seeds of barley, wheat and faba bean grown at the same site were compared. For barley, site of growth was the most important determinant of Mn content of the seed. Cultivar differences were not statistically significant. There was a significant season by site interaction which indicated that season affected seed Mn content at a site with low Mn availability but not at a site with adequate Mn. Manganese concentrations in seeds of different species grown at the same site varied considerably. 'Tatiara' wheat had more than twice the Mn concentration of 'Schooner' barley (48 and 21 mg kg-1, respectively), while small-seeded faba bean had the lowest Mn concentration of the three (6 mg kg-1). In white lupin, there were significant differences in seed Mn content of plants growing side by side at the same site. Significant within-plant variation was also found for both white lupins and barley. The range of Mn concentration of seed from one plant was 1530 to 3750 mg kg-1for white lupins and 1 1 to 24 mg kg-1 for barley. In barley (and probably most plants), variability of seed Mn concentration can be minimized by selecting seed by weight from parents grown at the same site during the same season. Variability of Mn concentration and content of white lupin seeds is not as easily accounted for and thus is more difficult to minimize. For barley, there was a positive relationship between seed weight and Mn concentration (r2= 0.66), while in white lupins, no such relationship was apparent. For white lupins with high concentrations of Mn, seed Mn variability could not be accounted for by genotype or seed weight.