Phenotypic plasticity allows organisms to cope with environmental variation and may aid in the evolution of novel traits. However, whether phenotypic plasticity is beneficial, or if acclimation responses might be constrained to particular ecotypes is generally poorly explored. Here we test the beneficial acclimation hypothesis (BAH) and its alternatives for desiccation resistance to atmospheric moisture in mesic- and xeric-adapted Glossina species. Highly significant interactions among acclimation and test humidity were detected for water loss rates indicative of significant phenotypic plasticity. Ordered-factor anova was unable to reject predictions of the 'drier is better' acclimation hypothesis in xeric Glossina morsitans and mesic G. austeni. Evidence for the 'deleterious acclimation hypothesis' was found for mesic G. palpalis as expected from the moist habitats it typically occupies. By contrast, support for the 'optimal acclimation hypothesis' was found in xeric G. pallidipes. Little support for BAH was obtained in the present study, although other hypotheses, which might enhance fitness within the environments these species are typically exposed to, were supported. However, acclimation responses were not necessarily constrained to xeric/mesic ecotypes which might be expected if adaptation to a particular environment arose as a trade-off between plastic responses and living in a particular habitat. These results highlight the complexity of acclimation responses and suggest an important role for phenotypic plasticity in moderating environmental effects on evolutionary fitness in Glossina.