The specific mechanisms of host plant resistance in alfalfa,Medicago sativaL., to the spotted alfalfa aphid,Therioaphis trifolli f. maculata(Buckton), remain unknown despite three decades of research. Difficulties with Painter's commonly used concept of host plant resistance were avoided when the Salt/Doutt paradigm of host selection by a parasite was used instead. Mechanisms of host plant resistance proposed in the literature were reviewed with respect to this alternative concept. Published results on the effects of host plant resistance are not comparable across different studies because of arbitrary measurements used. This problem is avoided with the use of laboratory age-specific life tables. In this study, four alfalfa cultivars were tested for host plant resistance to the spotted alfalfa aphid at three temperatures, and the statistical tools needed for the analysis are presented. Theintrinsic rate of natural increase(rm) was used as the measure of aphid performance on each cultivar. At the same temperature,rmvalues of the spotted alfalfa aphid on susceptible plants were significantly higher than on resistant cultivars. A developmental threshold of 7.9°C was estimated from the data, and the optimal temperature for growth and reproduction of this aphid was ≈21°C. Life tables of two successive generations of the spotted alfalfa aphid on a resistant cultivar indicated that aphids do not transmit the capacity to overcome resistance in a simple Mendelian fashion. A mechanism involving the random transmission of internal symbionts required by this aphid to overcome alfalfa resistance is hypothesized.