Abstract: The currant lettuce aphid, Nasonovia ribisnigri (Mosley) is the most common aphid species on field-grown lettuce in Europe. Cultivation of lettuce varieties, resistant to the currant lettuce aphid (possessing the resistance Nr:0), was an important tool to control Nasonovia in the crop, until the plant resistance was broken down. In 2007, the currant lettuce aphid evolved a new biotype, which overcomes the lettuce resistance gene Nr:0. This biotype is designated as biotype Nr:1. Its presence was recorded in North and Central Switzerland for the first time in 2008. The dispersion of biotype Nr:1 in these parts of Switzerland was monitored using biotests in a climate chamber. According to the results, the biotype Nr:1 colonized the main lettuce production areas and dispersed also to remote sites until 2010. In comparison to susceptible lettuce, the biotype Nr:1 showed a lower performance on Nr:0 resistant cultivars. Therefore, varieties conferring the Nr:0 resistance are useful to reduce the infestation by biotype Nr:1. Although the resistance is broken down, their cultivation is still recommended.