none Cold hardiness was examined on Lysiphlebus fabarum (Marshall). Trials were conducted to determine the optimum temperature conditions at which mummies of Aphis fabae Scop. parasitised by L. fabarum could be stored without any detrimental effect on the emergence of the adult parasitoid. The temperatures used were the following: -3 +- 1 degree C, 3 +- 1 degree C, 6 +- 1 degree C, 9 +- 1 degree C. Acclimatization as well as deacclimatization were carried out. The effect of cold storage on sex ratio as well as on adult reprodutive capacity was investigated. Test at temperatures of -3 +- 1 degree C gave no adult emergence even after short storage times. Mummies kept at 3 +- 1 degree C gave 92% of adult emergence after 1 week of storage, 48% after 2 weeks and no emergence after 3 weeks. The numerical difference between emergence from mummies subjected to 6 +- 1 degree C up to 128 hours (6 days) and the corresponding control groups was not significant, while a low number of adults emerged in the 256 (11 days) and in the 512 hours (22 days) groups. At 9 +- 1 degree C, however, cold storage did not produce any adverse effect up to 32 days. In our trials sex ratio of adults did not seem to be affected by cold storage, while the latter could have led to a certain decrease in fertility of the adults stage.