Several studies concerning the secondary compounds of plants and their relation to herbivory suggest that they may play an important role in host selection by herbivores. Qualitative and quantitative variation are present in different species varieties, and therefore, are supposed to be related to resistance to herbivores. It is known that saponins are responsible for the antinutritional effects related to deterrency as a result of their bitter taste to monogastric animals. They can also make complexes with cholesterol thus decreasing its availability in the diet and causing non-specific inhibition of several enzymes, such as proteases. Three alfalfa (Medicago sativa) varieties, P30 (susceptible), CUF 101 (resistant) and Crioula (most frequently cultivated in Brazil), with different levels of resistance to aphids were cultivated at Fazenda Canchim\CPPSE\EMBRAPA - Brazil and the number of aphids were counted monthly from September/97 to August/98. The average number of Therioaphis trifolii and Aphis craccivora aphids in the CUF 101 variety was significantly smaller than that presented by the two other varieties, which indicates a possible resistance to these species. Although CUF 101 had been previouly described as resistant to Acyrthosiphon spp aphids, that is A. pisum and A. kondoe, there were no significant differences in the number of such aphids among the three varieties. The saponin profile of the three varieties, obtained by using HPLC, was determined and a correlation between such profile and the resistance in the CUF 101 variety to the T. trifolii and A. craccivora aphids was observed. However, the results suggest few differences in the saponin profile and in the concentration of major components among the three studied varieties, which seems to indicate that they may not be related to resistance to T. trifolii and A. craccivora aphids. Index terms: aphids, resistance, herbivory, chromatography, saponins

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