We have beeh studying the caterpillar activity of Zeuzera pyrina L. on filbert trees of Tarragona. The activity percentatge show three considerable decreases in winter of 1984, 1985 during January to March and in August of 1984. There are two periods of great activity, these are from Setember to Desember, and fron May to July. We found a significant correlation between Z. Pyrina monthly activity and temperature monthly average, althoug there is a little dispersion between the cloud of points and the regression straigh line. According to the results of RADJABI i DANIALI (1970), we don't believe that the winter cold in Tarragona can affect so much on caterpillar populations of Z. Pyrina.