Plants transformed with a nucleotide sequence coding for a truncated RNA 2 replicase gene of the subgroup I strain of cucumber mosaic virus, Fny-CMV, are resistant to cucumber mosaic disease. Two resistant lines representing independent transformations in the original study have been propagated, and their progeny have been examined. Resistance to Fny-CMV was genetically integrated and was retained in the R4 generation. Fourteen subgroup I CMV strains, 6 strains uncharacterized as to subgroup, and 4 subgroup II CMV strains, as well as potato virus X, tobacco mosaic virus, tomato aspermy virus, tobacco etch virus, and peanut stunt virus were inoculated mechanically to R2 generation plants of these two lines. Resistance was found to 12 subgroup I CMV strains and to all of the subgroup-uncharacterized strains. No resistance was found to subgroup II CMV strains nor to the other viruses listed above. However, the two plant lines behaved differently in response to 3 subgroup I strains, K-CMV, PRC-CMV, and MB-CMV, where one line (5) was more resistant than the other (2). RNase protection analyses indicated that these 3 strains vary more in sequence from Fny-CMV than do the other subgroup I strains tested. Resistance was also shown when aphids were used to inoculate plants with either of 2 subgroup I CMV strains; no resistance was observed when aphids were used to transmit a subgroup II CMV strain. Inoculation of transgenic plants with pseudorecombinants produced with the RNAs derived from Fny-CMV to which the plants are resistant, and LS-CMV, a subgroup II strain to which they are susceptible, indicated that the resistance-breaking capacity of LS-CMV is a function of RNA 2.