Technical Abstract: Lettuce aphid, Nasonovia ribisnigri Mosley (Homoptera : Aphididae), is a major insect pest of lettuce, Lactuca sativa L, in many commercial lettuce productions areas around the world. Resistance to lettuce aphid was first reported in Lactuca virosa L. accession IVT 280 and characterized as complete, i.e., virtually no aphids survived, and genetically dominant to partial resistance in L. virosa accession IVT 273. Complete and partial resistances were conditioned by two alleles, Nr (complete resistance) and nr (partial resistance), but the genetic relationship to susceptibility was not reported. We previously reported two new potential sources of unique genes for resistance to lettuce aphid in Lactuca serriola L. accession PI 491093 and L. virosa PI 274378. Here we report on the genetic and phenotypic nature of resistance to lettuce aphid in these two wild lettuces. Resistance to lettuce aphid in PI 274378 is complete and allelic to complete resistance in IVT 280. Resistance to lettuce aphid in PI 491093 was partial, recessive to complete resistance in ‘Barcelona’ that was derived from IVT 280, but dominant to susceptibility in ‘Salinas’. We propose the revised gene symbols for complete resistance, NrC, and partial resistance, NrP, which was originally designated nr but is now the symbol for susceptibility to lettuce aphid. The dominance relationships among these three alleles are NrC (in IVT 280, ‘Barcelona’) > NrP (in PI 491093) > nr (in susceptible genotypes). Expression of partial resistance in PI 491093 was variable in controlled infestation tests, but in a naturally infested field test provided a potentially useful level of resistance to lettuce aphid. Partial resistance, where complete resistance has not been widely deployed, may either alone or as a component of integrated pest management delay or prevent emergence of lettuce aphid genotypes that overcome complete resistance.