Abstract While in most species the adult sex ratio is around 1:1, it can be strongly skewed in some species; some of this can be explained by ecological conditions and limits to dispersal. We hypothesize that stronger isolation imposed by ecological conditions leads to more pronounced female-biased sex ratios in the groundwater peracarid genus Ingolfiella Hansen, 1903. About 75% of all adults are female, and female-biased sex ratios are present in 30/42 of species for which individuals have been sexed. Sex ratios were not correlated with sexual size dimorphism. The adult sex ratio varied little between species found in different habitats (caves, beach sand, and seabed) thus not supporting our hypothesis that ecological conditions shape adult sex ratios. It appears that sediment structure in most habitats restrict ingolfiellids in their movement. Limited dispersal abilities and small mating assemblages may favour strongly female-biased sex ratios.