Laboratory populations of Tisbe clodiensis have been studied to provide information on the action of temperature on the fitness of some genotypes. All the populations were started with virgin females and males of the three genotypes (pp, pP, PP) at f(p) = 0.2 in agreement with Hardy-Weinberg distribution. Three of them were maintained at I5°C. and sampled at intervals of about 30 days, during a period of 273 days. Other populations have been kept at 18°C. and sampled at shorter intervals. The frequency of the allele p at 18°C. increased rapidly and after about 6 generations f(p) reached the value 0.6. In the populations kept at 15°C. after the same number of generations f(p) reached 0.4 only. These data show that the fitness of the gene considered can be influenced by temperature: in particular the dominant allele appears to be favoured by the lower temperature.