Abstract: In Europe, the polyphagous planthopper Hyalesthes obsoletus Signoret (Hemiptera,Cixiidae) is assumed to be the most important vector of the grapevine yellows disease bois noir,which is caused by phytoplasmas of the stolbur 16Sr-XII-A group. For a better understanding of theepidemiology of bois noir in Switzerland, distribution and host plant preferences of H. obsoletuswere studied in the field as well as in the laboratory. A national survey revealed that H. obsoletus ispresent in vineyards of southern, western and northern Switzerland; however, no specimens werecaught in the east of Switzerland, where evidence for the disease is lacking. Even though fieldbindweed (Convolvulus arvensis L.) was much more abundant in vineyards than stinging nettle(Urtica dioica L.), H. obsoletus adults were captured almost exclusively on the latter. A second,more targeted field study confirmed H. obsoletus' clear preference for U. dioica. From June toSeptember 2008, adults of H. obsoletus were primarily captured on U. dioica. However, fewindividuals were also caught on C. arvensis, hedge bindweed (Calystegia sepium L.) and four otherdicotyledons. Emergence traps set up above U. dioica and C. arvensis highlighted that significantlymore nymphs emerged from stinging nettle than field bindweed. Besides, a clear associationbetween the distribution of U. dioica patches and the incidences of bois noir infected vines could beobserved in a commercial vineyard. In conclusion, there is good evidence that H. obsoletus is alsothe most important insect vector of bois noir in Switzerland and that the insect prefers to feed anddevelop on U. dioica. Stinging nettle therefore plays a major role in the epidemiology of bois noirin Swiss vineyards.