Geckos are generally nocturnal and show no sexual dichromatism, and males are usually smaller than females. Ptyodactylus guttatus is an unusual gecko. It is active by day and night and is sexually dichromatic, and males are larger than females. We studied P. guttatus at two sites in southern Israel. Eggs were laid between May and October at Haluqim, where we studied eggs in natural caves. At a second site, Hazeva, P. guttatus occur on buildings at a density of 390 ha⁻¹. This is 195 times the density that the species occurs in natural habitats. At Hazeva, adult males inhabited large home ranges, which generally overlapped with one or more smaller home ranges of females. The pattern of home-range overlap presumably reflects the opportunities for mating. A path analysis was consistent with the hypothesis that females spaced themselves to gain access to insects (which they eat) attracted to wall lights on the buildings, and that males space themselves to gain access to female mates. We did not find evidence of territoriality in this population but observed males guarding females. Large male body size may provide priority access to mates, and the distinct male dorsal pattern may have arisen because light does not constrain the use of visual signals in these unusual, largely diurnal geckos, to the same extent as it may in nocturnal geckos.