For the great majority of the amphipods and isopods a biased sex ratio is attributed to photoperiod or to micro-organisms present in the cytoplasm of the oocytes. Since monogenous pairs are found in orders and species phylogenetically very far from each other, in order to try and clarify this phenomenon, two geographical populations of Asellus aquaticus (Isopoda) were collected in the Netherlands and in Italy, where the duration of the cold season and the photoperiods are very different. From these parental (P) populations, 200 females and 200 males per population were randomly subsampled and bred under standard conditions of temperature and nutrition. One half of each P generation was subjected to 18 hours light per day, the other to 14 hours light per day. New-born of each pair (laboratory F1) were grown up to differentiation of external sexual characters under the same photoperiod experienced by the parents. Also, hybrid F1 generation, born from mating between the two populations, was conceived in both photoperiods, but, after birth, one half of the new-born was maintained in the same photoperiod in which they were conceived, the other half was grown under the other photoperiod. No significant difference between the sex ratios was found in the two photoperiods, neither between Italian nor between Dutch Asellus. The sex ratio of Dutch F1 is female biased, while it is male biased in Italian Asellus. The female- or male-biased sex ratio can be ascribed to the high proportion of monogenous pairs in which offspring sex ratio is significantly biased towards females (in the Dutch population) and/or in which offspring sex ratio is significantly biased towards males (in the Italian population). On the basis of these results we can rule out the influence of photoperiod in sex determination for this species. The results shown by the hybrids suggest some form of maternal inheritance. In fact, the hybrids' sex ratio as indeed the frequency of pairs breeding one sex alone, was skewed towards the same sex for which the maternal population showed a bias. We therefore consider the possibility of sex determination associated with a cytoplasmic factor (a mitochondrial DNA?), which would inactivate only one of the two sets of genes governing sex determination.