American grey squirrels Sciurus carolinensis introduced to northern Italy in 1948 have caused damage to commercial poplar plantations and have replaced the native red squirrel Sciurus vulgaris from most of the 350 km² of the Piedmont Po-plain they currently occupy. In order to plan a control programme aiming to stop grey squirrels from further spreading and to decrease their numbers, the current distribution and population size in the highly fragmented landscape of the Po-plain were studied. The probability of finding grey squirrels in woodland fragments increased with habitat quality (diversity of trees producing large, consumable seeds), woodlot size and the proportion of poplar. Adding isolation variables did not improve the fit of the logistic regression model that predicted squirrel presence. The density of squirrel dreys, an index of population density, in the large Stupinigi forest also increased with tree species diversity. An estimate of the minimum population size for all woodlots assessed for squirrel presence was 1,260 animals in the summer of 1996. This extrapolates to a total of ca 2,500 grey squirrels in Piedmont. Grey squirrels continue to increase their range and are getting close to the continuous mixed forests of the pre-Alps and to large hazel plantations. Control measures to stop the spread of grey squirrels, and eventually to eradicate them, should be implemented immediately.