Summary: The Arlington line of cowpea (Vigna unguiculata) is known to show extreme resistance to Cowpea mosaic virus (CPMV, Comovirus genus) with no virus recovered and no observed symptoms. The resistance is inherited under control of a dominant locus designated Cpa. Nearly isogenic Cpa/Cpa and cpa/cpa cowpea lines are available from a backcross series between lines Blackeye 5 (genotype cpa/cpa) as the recurrent parent and Arlington. Coinoculation of Cpa/Cpa seedlings with any of several cowpea-infecting viruses and CPMV resulted in interference with accumulation of the former. Both extreme resistance and the interference phenomena were known to be controlled by the larger of two CPMV genomic RNAs, RNA1. The experiments described in this dissertation were aimed at identifying the CPMV RNA 1-encoded elicitor of extreme resistance. RNA1 sequences were tested with two transient expression systems. CPMV RNA1-derived segments were delivered by Tomato bushy stunt virus vectors into Cpa/Cpa and cpa/cpa cowpea. Only constructs containing the entire 24KPro sequence induced a differential on these two cowpea lines: necrotic lesions on Cpa/Cpa cowpea and chlorotic lesions on cpa/cpa cowpea. The 24KPro constructs derived from three other members in Comovirus genus, Bean pod mottle virus, Red clover mosaic virus and Cowpea severe mosaic virus, did not induce the differential response. The results using TBSV vectors showed that CPMV 24KPro is the likely elicitor of Cpa-mediated extreme resistance to CPMV and that the 24KPro enzymatic activity is necessary to the elicitation. The CPMV 24KPro elicitation was also tested by pressure infiltration of transformed Agrobacterium tumefaciens (agroinfiltration). Only the intact CPMV wildtype 24KPro sequence induced a hypersensitive response (HR) in Cpa/Cpa cowpea, a response not seen for agroinfiltrated cpa/cpa cowpea. Constructs in which alanine was substituted for any of the three amino acid residues (cysteine, glutamic acid and histidine) of the 24KPro catalytic triad failed to give the necrotic reaction on Cpa/Cpa cowpea, for both transient expression systems. Thus, results from TBSV infection and agroinfiltration were in agreement, but results from the latter were more definitive and more easily interpreted. Both implicate the activity of 24KPro as being essential to Cpa-genotype cowpea recognition of CPMV infection.