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Environmental fluctuations often select for adaptations such as diapause states, allowing species to outlive harsh conditions. The natural sugar trehalose which provides both cryo- and desiccation-protection, has been found in diapause stages of diverse taxa. Here, we hypothesize that trehalose deposition in resting stages is a locally adapted trait, with higher concentrations produced in harsher habitats. We used resting stages, produced under standardized conditions, by 37 genotypes of; Daphnia magna; collected from Western Palaearctic habitats varying in their propensity to dry in summer and freeze in winter. Resting eggs produced by; D. magna; from populations from summer-dry habitats showed significantly higher trehalose than those from summer-wet habitats, suggesting that trehalose has a protective function during desiccation. By contrast, winter-freezing did not explain variation in trehalose content. Adaptations to droughts are important, as summer dryness of water bodies is foreseen to increase with ongoing climate change.