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Although lichens are important components of arid ecosystems, the influence of microclimatic factors (particularly dew) conducive to their distribution on rock habitats in the Negev Desert are still not sufficiently investigated. Here, we present a study on adjacent lichen communities inhabiting slightly acidic flint (chert) and slightly alkaline limestone cobbles in the north-facing slopes of two sites in the Negev Desert, Nizzana, NIZ (with long-term mean precipitation of 95 mm and average daily dew amount of 0.1 mm) and Sede Boger, SB (with long-term mean precipitation of 95 mm and average daily dew amount of 0.2 mm). The communities on flint consisted of either scarce or lush cover of exclusively epilithic species, while endolithic lichens predominated on limestone. Following previous measurements that indicated that similar dew amounts characterize limestone and flint cobbles, we hypothesized that differences in the flint- and limestone-lichen communities may stem from differences in dew duration. Our results pointed indeed to the fact that while non-significant differences characterized the amounts of dew condensed on both substrates, dew duration on some of the flint cobbles (such as flat cobbles that readily permits long-lasting condensation) was by similar to 1.2 longer. With longer dew duration, the cover of the epilithic lichens Aspicilia contorta subsp. hoffmanniana and Buellia sorediosa on flint increased along with the cover of the fruticose lichen Ramalina maciformis. Some of the species (Caloplaca circumalbata var. circumalbata, Caloplaca flavocitrina, Caloplaca oasis, Candelariella minuta, Diplotomma epipolium) inhabited both substrates pointing to the possibility that dew duration rather than alkalinity determines their distribution. While not explaining the higher lichen cover on limestone, dew duration explains the significantly higher cover of epilithic lichens and chlorophyll content (per inhabited surface) on flint in comparison to limestone. (C) 2015 Elsevier GmbH. All rights reserved.