Alfalfa,Medicago salimL., cultivars ‘Lahontan,’ ‘Hayden,’ ‘UC Cargo,’ and ‘Caliverde 65’ were screened and evaluated for resistance to biotypes Band H of the spotted alfalfa aphid (SAA),Therioaphis maculata(Buckton) (Homoptera: Aphididae). Surviving plants screened by biotype B or H were evaluated simultaneously for resistance to both biotypes, then were rated by their response to each biotype. There was a significant differential resistance to biotype B among cultivars screened by biotype B. Some of these plants in each cultivar were resistant to biotype H, thus showing combined resistance to both biotypes. These results suggested there was a number of selected plants from each cultivar that expressed genes for horizontal resistance. There was no differential resistance to biotype H among cultivars that had been screened by biotype H, but all surviving plants resistant to biotype H also were resistant to biotype B. Screening by biotype H detected significantly more ‘Lahontan’ plants having combined resistance than did screening by biotype B. Vertical resistance to SAA was demonstrated in the cultivars ‘Moapa,’ ‘Moapa 69,’ ‘Sonora,’ ‘Sonora 70,’ ‘Mesa-Sirsa,’ and experimental-line MSTT. Horizontal resistance to five SAA biotypes was demonstrated in three parent clones and in ca. 17% of the progeny of ‘Lahontan.’ We found an indication of horizontal resistance in ‘Mesa-Sirsa’ and MSTT.