Using Krumbein's visual comparison chart, a study is made of the effect of day of observation, observer and sample size on the roundness of quartz grains, using medium, fine and very fine sand as obtained from the "arenito Botucatu" soils in the region of Piracicaba, SP, Brazil. As to sample si ze, 25 grains proved to be sufficient. No significant statistical effect was observed for day of observation. The effect of observer was verified for medium and fine fractions only.