The general belief that dune sands in arid regions are not likely to generate runoff and that runoff necessitates wet conditions is examined. Runoff generation was measured within an arid dune field in the western Negev Desert, Israel, during 1990--1994 on 16 plots (1.2--6.6 m2), constructed along a continuum of crust-covered crust. The relationships between chlorophyll a and carbohydrates of the microbiotic crust and runoff yield were studied. Whereas chlorophyll a (15--45 mg m-2) and carbohydrates (4.9--17.6 g m-2) of the cyanobacterial crusts showed positive linear relationships with runoff coefficients, a significant reduction in runoff yield was obtained once the moss-dominated crust (with 53--54.5 mg m-2 chlorophyll a and 28.5--28.9 g m-2 of carbohydrates), which inhabited the wettest habitat within the dune field, was also included in the analysis. Contrary to previous assumptions that an increase in runoff generation occurs on arid and semiarid sand dunes in wet climates, wet climates may lead to the establishment of moss-dominated crusts that in turn decrease runoff yield.