A carlavirus was isolated from naturally-infected dandelions in the Okanagan Valley, B.C. In total, 31 plant species belonging to 12 families were tested as possible hosts for the dandelion virus. In only four families (Amaranthaceae, Chenopodiaceae, Compositae, Solanaceae) were susceptible species found. The virus was contained as local lesions in Gomphrena globosa and Datura stramonium and became systemic in Chenopodium amaranticolor, C. quinoa and Taraxacum officinale. The carlavirus, for which the name Dandelion Virus S (DVS) is proposed, has slightly curved particles with normal length 637 nm and width 12-13 nm. A purification scheme was developed that yielded 20-30 mg of virus per kg of C. quinoa leaf and stem tissue. Partially purified virus preparations had a single nucleoprotein component in rate zonal sucrose and cesium chloride density gradient centrifugation. The UV absorption spectrum has a maximum at 259 nm and a minimum at 245 nm. The ratio, of A[sub max]/A[sub min] is approximately 1.1; of A₂₆₀/A₂₈₀,1.4. In sap from infected C. quinoa, DVS had a thermal inactivation point of 75-80°; an infectivity dilution end point of 2 x 10⁻⁵ to 2 x 10⁻⁶ ; a longevity in vitro of 4-5 days at 23°, 28-56 days at 4° and at least 16.5 months in a lyophilized state at 23°. An antiserum against DVS was prepared by four intramuscular injections of 1 mg each and the maximum homologous titre was 40,960. Two carlaviruses with similar symptoms in C. quinoa, Peru virus S (PeVS) and Helenium virus S (HVS) were purified for antisera production and comparative serological testing. Antisera to other carlaviruses were also used to determine if serological relationships existed with DVS and other members of the group. Serologically, DVS is related to potato virus S (PVS) and PeVS, and distantly related to chrysanthemum virus B (CVB), Helenium virus S (HVS) and narcissus latent virus (NLV). / Land and Food Systems, Faculty of / Graduate