Since its introduction in Italy in 1990, Aedes albopictus has spread quickly across the northern and central regions of the country. The Italian populations of the species probably originated from temperate areas and are able to survive the cold season through the production of diapausing eggs. In 1997, Ae. albopictus was detected in Rome, where it seems to have found ideal environmental conditions for proliferating and extending its season of activity. In 2000, we carried out a study to evaluate the length of the season favorable to the species in Rome and the factors that might induce production of diapausing eggs. Adults of the species were reported to be active from February-March to December, peaking in August-September. Female Ae. albopictus produced different numbers of summer eggs throughout the study. The peak of egg hatching occurred at the 35th week of the year (end of August). Starting from the 36th week, when the photoperiod dropped below 14:10 h light:dark, a rapid decrease in summer egg production was recorded; the minimum value (17%) was reached during the 1st week of October. A new rise in egg hatching was recorded in October-November, probably because of a polymorphism in the diapause response of the population of Ae. albopictus and to the persistence of mild temperatures up to the end of November. Eggs completely ceased to hatch in mid-December.