Salinity tolerance of rice varieties was determined by comparative measurement of seedling root growth in saline and in non-saline conditions. A tolerance ratio (TR) of root growth after four days in 80 mM NaCl solution to root growth after four days in normal culture solution was calculated for each individual within a variety. This technique was used to screen 1058 varieties/advanced lines available at the WARDA-Rokupr Station and was found to be reliable, rapid and simple. Substantial variation in seedling tolerance to salinity was found within and between varieties and populations bearing the same variety name. The seedling TR was correlated with the visual scores (r = 0.80, df = 1058, P < 0.001) of salt symptoms and survival (r = 0.75, df = 1058, P < 0.001) at 80 mM NaCl solution. The scores after four weeks of plant exposure to 80 mM NaCl confirm the ranking of the varieties on the basis of the TR. Whilst visual scores can be subjective, the TRs were obtained from root length which is a quantitative characteristic. Humidity and temperature appeared to affect the germination and rate of root growth, thereby affecting the TR. The changes in TR due to seasonal drift were overcome by adjusting the TRs of all test varieties with the TR of the tolerant check.