Seeds of Eucalyptus mucalata Hook were classified as to size making use of round hole screens of 2.00, 1.68, 1.41, and 1.19mm of diameter. The fraction smaller than 1.19mm showed a high proportion of chaff and was thus discarded. In each one of the size classes seed weight and number were determined and the results expressed as percentage of the total weight and number. Seed purity and 1,000 seeds weight were also determined. The physiological quality of the seeds was determined through germination and vigor (speed of germination and seedling dry matter content) tests. The results showed that seeds larger than 1.68mm were the most abundant and displayed the highest physical and physiological quality. Seeds larger and smaller than 1.68mm should not be mixed at seeding time.