Abstract: The aim of this study was to compare the resistance of 33 alfalfa varieties to thrips, and to clarify the relationship between population density of thrips and alfalfa plant height and plant damage index. The thrip population of a single branch of each of the 33 varieties was counted during flower development and the main flowering period of the 2^nd and 3^rd alfalfa cuts of a growing season. Plant height and severity of damage to leaves of the sampled branch were also investigated in the primary flowering period prior to the 3^rd cut. Thrip resistance of the alfalfa varieties was compared using a damage index and fuzzy clustering techniques. The relationships between population density of thrips and damage index and plant height were analyzed by regression. All alfalfa varieties carried thrips with thrip density ranging from 3.15-8.55 thrips/head. As assessed by damage index, the most resistant variety was Bararlfa 53HQ, and the least resistant was 5S43. Using Fuzzy cluster analysis, the most resistant variety was Baralfa 421Q, while the least resistant variety was again 5S43. Despite some ranking differences between the two methods, the general conclusions were the same. Since cluster analysis uses population density and damage grade of leaves, it is more suitable for evaluation of resistance. The equation slinking population density and damage index and branch height were, respectively, y=(-113.2563+53.6489x)/x, and y=x/(-0.0104+0.0230x). A chi-square test (χ²) showed the two equations to be related.