Strong resistance to accumulation of potato leafroll virus (PLRV) was identified in a clone of the diploid potato species Solanum phureja (cv.'Egg Yolk', clone 5010) using a quantitative enzyme-linked immunosorbent assay (ELISA). The resistance is expressed very strongly in leaf tissue (virus could not be detected in leaves of some plants although other tissues were infected) but less strongly in petiole and stem tissue of infected plants. The titre of PLRV in leaves of S. phureja (5010) is approximately 2%, or less, of the titre in S. tuberosum cv. Maris