Trichosirocalus (= Ceutorhynchidius) horridus (Panzer), introduced from Italy into the USA for the control of Carduus thistles in 1974/75, is well established in Virginia (Kok and Trumble 1979). Field studies on the development of this weevil on Carduus thistles in Italy (K. E. Frick, USDA, Stoneville, Mississippi — pers. comm; Boldt and Campobasso 1981) and in Virginia (Trumble and Kok 1979) revealed that weevil eggs were commonly found in rosettes during the winter months. In Virginia, first instars were detected in rosettes immediately after the snow melted (Trumble and Kok 1979). Therefore, the eggs are exposed to the coldest temperatures in January and February when air temperatures reach −20°C in western Virginia. The effects of such temperatures on the proportion of hatch and on the viability of the hatchlings are not known. In addition, T. horridus has consistently laid the maximum number of eggs in November and December in the laboratory for the past 10 years. Thus, a large supply of thistles had to be grown in the greenhouse to support the weevil population during the winter when plant growth was poor and mortality of laboratory reared T. horridus was high. In any successful biological control program the proper time of release of the beneficial organism as well as numbers available is critical. Successful storage of T. horridus eggs would greatly increase the number of T. horridus available when needed for inoculative releases or experimental purposes. The purpose of this study was to determine suitable temperatures and storage times for T. horridus eggs without affecting hatch and viability.